If you need to tell someone to do keyboard commands you can use the kbd HTML tag.

Source

<kbd>Ctrl</kbd> + <kbd>Alt</kbd> + <kbd>Del</kbd>

Result

Ctrl + Alt + Del

Attached Firefox/Internet Explorer search plugin. In Firefox, when you are on Stack Overflow, just click on the search dropdown (to the left of the search text field) and hit Add "Stack Overflow".

EDIT: SO-family sites now also support OpenSearch, as described in this question ^[1].

Clicking on Days Visited or Consecutive Days on the profile page

opens a calendar that shows which days the site was actually visited.

You can use:

http://stackoverflow.com/reputation

All the votes ( negatives and positives ) you have had, and the question where you've got them.

Source: Jeff answer here ^[1]

URL tooltips ^[1]

You can see the history ^[1] of what happened to a question by going to:

http://stackoverflow.com/posts/{questionid}/timeline

In the details that are shown ^[2], a lot can be clicked to expand.

This also works on Area 51 ^[3].

You can see recently completed features ^[1] on Meta Stack Overflow. There is also a formally-maintained list ^[2].

If you have 1000+ reputation, you can click the score of a question/answer and expand the net value to see the total of upvotes vs total of downvotes

Source ^[1]

#stackoverflow ^[1] IRC channel on freenode, and the SObot that will post new questions in the channel as they're posted.

You can search for multiple tags by typing them in the URL separated by %20. This does an AND search, i.e. only questions with all the tags you specified will be shown.

For example:

http://stackoverflow.com/questions/tagged/windows%20com

searches for both the 'windows' and the 'com' tags

Alternatively you can type the following in the search box in the top right for the same results:

[windows] [com]

The nofollow attribute on your profile website link is removed at a rep of 2000.

Jeff's answer ^[1]

Navigate to the last modification of a post

I recently learned ^[1] that the Your Question/Your Answer editor has an auto-indent feature (in source code blocks): Try Shift+Enter.

It's possible to view the markup used to create any question or answer. (This may be useful if you wish to combine answers or quote part of the question with formatting.)

If the question or answer has been edited so that it has an edited X timeunits ago link, you can use that to view the revision history, then use the view source link.

If it hasn't been edited then you can still view the revision history, but you'll have to manually enter the url in your browser's address bar. For questions, take the number in the middle of the page's url; for answers use the link link for the answer to get it's unique number in the anchor of the url (after the # character) and then go to:

http://stackoverflow.com/posts/number/revisions

Wildcards in tag filters ^[1]

Adding *C#* to your ignored tags will ignore any tag that contains the letters C#, including C#-4.0 and visual-C#-express-2010. Likewise for favorite tags.

This is useful because there are often questions tagged C#-3.0 but not C#, so just ignoring C# wouldn't catch this. Its a really nice solution to ignoring all questions on a given topic.

You can even use it to ignore all questions not in your favorite tags by just ignoring * :D

Tag links:

bug ^[1]

Syntax: [tag:tag-name]

Meta tags:

status-planned ^[2] (red because it's moderator only tag)

Syntax: [meta-tag:tag-name]

I just noticed today that asker comments are shaded just like the background for the user box:

This applies in comments to everything for that particular question too. It can be handy for context on larger pages.

Modify tags in question (hidden area at right of tag list):

Note: This is only available with 10k rep or more.

Well, not only related to Stack Overflow but also AWESOME: ODATA for all Stack Exchange sites ^[1].

There's even some saved queries to know. For example, how much is still missing for me to get the X badge.

This is awesome!

Stack Overflow Meta ^[1]

All the editing keyboard shortcuts act as toggles, including add hyperlink.

When you search for a popular keyword ^[1] the search gets auto-converted to a tag search.

hibernate ^[2] - results: 6,560 questions tagged hibernate ^[3]

You can override that by prefixing a plus sign.

+hibernate ^[4] - results: 4,807 search results for posts containing +hibernate ^{[5]
(some of which are not tagged hibernate [6])}

Typing --- in chat forces a new section. Usually, consecutive chat messages from the same user are kept in the same box.

It is hidden in the transcript, but the separation between sections is still visible:

You can write small text using <sub><sup> trick. For example wrapped with "single" <sub><sup>:

_{^{hello world}}

With two:

_{^{hello world}}

Every site on Stack Exchange network has a "network login help" page, which run a few tests to check for the availability of certain necessary features in the user's browser.

To reach the page just append /network-login-help to the URL of any site, e.g. for the main site: http://stackoverflow.com/network-login-help

Comment timestamp is a permalink to that comment: (need to right click it --> copy link address)

In the above example clicking that timestamp leads here ^[1].

On the personal profile page there is a Tags list just before Badges. There, some numbers are written before and after the tag, like

162 php ^[1] × 302

When you hover on that tag you can see the total statistics of

• How many questions you have asked within that tag
• How many questions you have answered in that tag and
• How many upvotes you have received in that tag

For example, if I hover on the php ^[2] tag, I see these on the tooltip:

Asked 71 non-wiki questions with a total score of 56. Answered 230 non-wiki questions with a total score of 162.

302 is the sum of total questions and answers in the php ^[3] tag
162 is the sum of votes for answers in the php ^[4] tag

UPDATE

Recently I found one more hidden feature.

In the comment box, you can not notify more than one user, that is, you can not use @ more than once (until it is inside tilde). Doing so, you will get the warning below.

Only one additional @user can be notified; the post owner will always be notified

While in the /about page aka guided tour of any site (e.g. Stack Overflow about page ^[1]) in the network, press the following keys in the following order:
upupdowndownleftrightleftrightBAenter.
This is known as Konami code, and will cause the site icon on top of the page to become the Stack Exchange Venn Euler diagram ^[2].

If on mobile device, the following should work: tap-drag up, tap-drag up (,down,down,left,right,left,right), and then tap thrice.

Kudos to Geoff for revealing the existence ^[3] of that easter egg. :)

You can determine the total number of active users by looking at how many people have the "Teacher" badge (posted at least one upvoted answer):

http://stackoverflow.com/badges/1/teacher

Start a blockquote with >! for:

spoilers!

View other users' privileges page by visiting /help/privileges/user/<id> URL. This page is accessible without any restrictions which seems to be by design ^[1].

^{Came to know about this feature from a post made by animuson [2].}

You can get a list of all of the questions and answers for which you've gained or lost reputation (along with the points up or down) in JSON format by pointing your browser at:

http://stackoverflow.com/users/###/0/9999999999999

Substitute your user number for the ### in the URL. The 0 and 9999999999999 are the starting and ending times for the report in milliseconds (so this goes from time zero to several hundred years).

[Jun2009] Edit: This feature is apparently no longer supported, or the format has changed. Sorry.

Associating your Stack Overflow login with another Stack Exchange site gives you +100 free reputation, just like that. I only just discovered this when I decided to set up a leatherworking site on Area 51 ^[1], but there's nothing I can find about it in the FAQ or elsewhere. I think they wanted to make sure people weren't motivated just by the reputation, but here I go letting the cat out of the bag. :)

Quick access to see user's review stats: daily, weekly, monthly and total, plus last time he/she reviewed.

URL format is: http://stackoverflow.com/review/user-info/[review queue ID]/[user ID]

Review queue ID list:

1. Suggested Edits
2. Close Votes
3. Low Quality Posts
4. First Posts
5. Late Answers
6. Reopen Votes
7. ? (future use, or maybe available on specific non SO sites only)
8. Broken Links
9. ? (future use, or maybe available on specific non SO sites only)

Some examples, using Geoff as model:

• http://stackoverflow.com/review/user-info/1/2
• http://stackoverflow.com/review/user-info/2/2
• http://stackoverflow.com/review/user-info/3/2

This is just raw data, unformatted and of course can be changed or taken down at any point.

I can't believe no one has mentioned descriptions of Stack Exchange functions and data hidden mostly in tooltips. This came up most recently in this question about privilege percentages ^[1]. @Arjan commented "As always on these sites: when in doubt, hover your mouse over the text for more detail."

If you click on a question that was migrated, it takes you to the question on the site of migration. If you then click on the "migrated from blahblahblah.com" it will then take you back to the question stub on the first site.

It's not a very hidden feature that, on Stack Overflow, the HTML <title> of each question page will have the most popular tag prepended.

What is hidden is that if the person names the question with the tag anywhere in the title:

[Java] How do I do this thing

or

How do I do this thing in Java

It will not also prepend java - at the beginning of the HTML title!

While I answered this question in Stack Overflow: How does the Stack Overflow Search Input work? ^[1]

I found this hidden feature: If you type your search expression than clicking Ctrl + Enter it will open a new tab\window with the results for your search expression.

For some browsers you will need to allow popups in the site to make it work.

For the sake of completeness The keydown (javascript - jQuery)code looks like this:

function(c) {
    if (13 == c.which && c.ctrlKey && d.val()) 
        return window.open("/search?q=" + encodeURIComponent(d.val()), "_newtab"), !1
}​

Comment short links are now available: http://[site]/posts/comments/[id]

To obtain comment id you can extract it from the full comment link or use dev tools to inspect the HTML.

For example: http://meta.stackoverflow.com/posts/comments/14689

http://stackoverflow.com/review

Shows recently upvoted answers from new users or users with low rep.

If you hover a username in the comments section of a question you get to see his (or her) reputation.

You can get raw view of post comments (either question or answer) by typing such URL:

http://meta.stackoverflow.com/posts/[post id]/comments

For example comments on this very question: http://meta.stackoverflow.com/posts/8211/comments

Working on all sites of course.

On per-site-metas, there is a tab for showing users by participation ^[1]. According to the tooltip it shows: Users most active in the last 60 days (combined number of posts, votes, comments and edits).

The URL ^[2] for this tab is http://meta.[site].stackexchange.com/users?tab=participation.

The main site does not have such a tab. But simply by removing the meta. part from the above URL, changing it to http://[site].stackexchange.com/users?tab=participation you get another list of users. Although it does not seem to be documented anywhere, it is quite probable that it orders users by activity on the main.

It works even for Meta Stack Overflow (which, as it is not a per-site-meta, does have reputation tab and not participation tab in the list of users): http://meta.stackoverflow.com/users?tab=participation.

Shortcut to the Advanced Search Options

Just search for an empty (or 1 character length) query and you will see the page.

Note: it is not recommended to link to this URL, use it for personal use only as this method of accessing might disappear in the future leaving a bunch of broken links.

You can browse directly to answers you have downvoted using a hidden feature of the stackoverflow.com/questions ^[1] page if you have the answer's unique ID (available on the Reputation Calculator ^[2] page).

(NOTE: Why you would do this anymore given that the answers are now hyperlinked directly from the Reputation tab is questionable, but it still works anyway.)

1. Browse to the Reputation Calculator ^[3] and locate a line representing an answer vote.

   They look like:

   3 XXXXXX (-1)

   where XXXXXX is the answer's unique identifier.

2. Browse to http://stackoverflow.com/questions/XXXXXX where XXXXXX is that answer identifier.

3. You will be redirected to another URL in the format http://stackoverflow.com/questions/YYYYYY/name-of-the-question-here/XXXXXX#XXXXXX

   ...which is a link to the answer that you downvoted on the question YYYYYY. That URL you've been directed to is also the one you'll see if you hover over the "link" hyperlink on the answer that you downvoted.

(This actually works for any answer as long as you know the id XXXXXX, not just ones that you've downvoted.)

You can run queries against the database for any Stack Exchange site with the Stack Exchange Data Explorer ^[1].

I did not know about this feature, or see it linked, until I read a Meta Stack Overflow post which used this example.

Query for Most Common Comments on Stack Overflow. ^[2]

*Although now I know it exists. I've found the link in the footer.

View all linked questions of any post using the format: http://stackoverflow.com/questions/linked/<post_id>

These could be further sorted as:

1. Hot
2. Newest
3. Votes
4. Active
5. Unanswered

For example http://stackoverflow.com/questions/linked/3102819 shows a page having all questions linked to/from " Chrome: Disable same origin policy ^[1]" post.

I was going to post a request asking for questions determined to be duplicates to be listed on the original question, but I found that there was already a feature request for it ^[1] and that it was status-completed ^[2] because you can find questions that are duplicates of this one by looking at the "Linked" sidebar ^[3]. Cool!

(Note: also appearing there, are questions, comments, etc. that link to this question.)

Putting a parameter ?lastactivity at the end of the question's URL will redirect you to the last activated (modified) post.

E.g. http://meta.stackoverflow.com/questions/8211/?lastactivity

Shortening questions and answers URLs ^[1]

Let's assume the following URL to answer:

http://stackoverflow.com/questions/5988444/why-is-the-llvm-execution-engine-faster-than-compiled-code/7250716#7250716

We can delete the middle text part, change the question to 'q' and leave only the answer number and still have a working URL : :)

http://stackoverflow.com/q/7250716

Just try it! ^[2]

While this way is not recommended, it is supported and works - according to what @genesis wrote ^[3].