I am looking for a good personal desktop wiki with a good desktop integration. Like for example drag and drop of files, images, etc. I am currently evaluating WikidPad .
Which personal wiki are you using and what is your experience with it?
Not a true wiki but it is excellent for gathering notes from various sources. http://evernote.com/
There is Zim , which is a desktop wiki. It is a very simple wiki, as it saves all its data in wiki-formatted text files, but it may be useful for you. http://zim-wiki.org/index.html
+1 for WikidPad. I used WikidPad extensively at work until coworkers and I grass-rooted a proper Wiki into the department. WikidPad is brilliant for simple-and-fast single user wiki work, and its formatting is easy to migrate to proper wikis later on. I also like that it can generate HTML output.
Alternatively, I highly recommend Foswiki.org (former TWiki). It's a highly mature Wiki with a very good WYSIWYG editor as well as solid wiki-ML, and it also allows all HTML tags! It is very easy to install on many platforms and contains good security measures.
DokuWiki is a standards compliant, simple to use Wiki, mainly aimed at creating documentation of any kind. It is targeted at developer teams, workgroups and small companies. It has a simple but powerful syntax which makes sure the datafiles remain readable outside the Wiki and eases the creation of structured texts. All data is stored in plain text files – no database is required.
+1 For TiddlyWiki
I will also throw in a shout out to GTDTiddlyWiki , which I believe is an offshoot of TiddlyWiki. GTDTiddlyWiki is a great personal organizer and personal information manager. http://nathanbowers.com/gtdtw/
I use TomBoy on Gnome now, as it comes with ubuntu gnome. Before (on KDE) I used and loved Basket http://basket.kde.org/.
While these may seem to fall outside the category "personal desktop wiki" I would argue they are just that. Or can be used as such.
Tomboy for example allows CamelCase and other wikiSyntax tricks to link to (new) notes or to insert things as dates. Trough plugins you can link to mails, include files, link to track-bugs and so on.
If you follow WikidPad development you will soon notice that is not realiable. I would stick to ConnectedText that has all standard wiki features (many missing in WikidPad) and has many extensions not available in other systems.
I've found Wiki on a stick  to be quite useful - like TiddlyWiki, it is zero config as everything is contained in the one file. http://stickwiki.sourceforge.net/
I haven't tried any of them yet, but I would probably go with MoinMoin Desktop Edition
Microsoft OneNote  for the win!
I use this for university, home, notes, things I need to store temporarily, drafts and so on... It has everything I need and even a lot more than that, it has browser, printer and Outlook integration. You can open a new note, a screen shot or OneNote itself with a windows hotkey. Searching, checking the history of what you have done, tagging things and finding those back and more... And many many more...
If you have Word and Excel, you will probably have OneNote installed already... ;-)
If I haven't convinced you yet here is a great article  about how efficient OneNote can be.
Take a look at Hatta Wiki .
It is based on python + mercurial, which means all changes are backed by a revision control system. http://hatta.sheep.art.pl/
We use ScrewTurn Wiki  in our department which is a free and easy one to use. http://www.screwturn.eu/
You should look at ConnectedText . It has all standard wiki features, it is robust and included semantic extensions, not available in other systems. http://www.connectedtext.com
LionWiki  is probably not what you need, but I still think it's worth noting - I use it to my full satisfaction. It's dead simple, practically zero configuration and just one script. http://lionwiki.0o.cz/
I recommend TiddlyWiki in combination with DropBox, since the tiddlywiki is just a java applet running through an html file, you can effectively use it anywhere with java installed. Combined with dropbox, this becomes a great tool for keeping notes.
I use The Guide . Windows only, open source, lightning fast. http://theguide.sourceforge.net/
You could look at Backpack by 37 signals with a desktop client. Backpack is not a true wiki but it does a lot of the things you might want.
You could take a look at VoodooPad  for Mac OS X. Really great application with good support. http://flyingmeat.com/voodoopad/
WikidPad is the way to go, it has plugins, like for writing LaTeX formulas etc. I've found it to be stable, easy to use, and feature-rich.
Zim is decent as well. Gnote or Tomboy are also okay. Be aware that since TiddlyWiki is just one file, it could begin to slow down if you store massive information in it.
Org-mode  for emacs. I know, it's emacs, but it's in a pure plain text and has a ton of extra features that are sometimes huge timesavers: time stamping, publishing as a website, agenda, etc. http://orgmode.org/worg/org-tutorials/orgtutorial_dto.php