Poll: What software must you always keep handy? I don't care if it is open source, freeware, or demo, as long as its price is $0.
Neither do I care if it is for desktops, handhelds, netbooks, web based, cellphones. If it is free to use – and essential to your happiness and well-being – put it in this list.
Google Chrome 
Google Chrome is a browser that combines a minimal design with sophisticated technology to make the web faster, safer, and easier.
Putty and all its related tools is crucial for SSH usage in Windows.
Process Explorer  is a must-have Task Manager replacement. http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
TrueCrypt  http://www.truecrypt.org/
Paint.net  : freeware image editing. http://www.getpaint.net/index.html
Adblock Plus  http://adblockplus.org/
FileZilla : Free FTP client, I use this on my Mac and Windows machines, nothing beats it in my opinion. http://filezilla-project.org/
TortoiseSVN  is a really easy to use revision control / version control / source control program for Windows. Since it's not integrated into a specific IDE you can use it with whatever development tools you like. http://tortoisesvn.tigris.org/
ccleaner  to remove all of the junk that I didn't want to save in the first place. http://www.ccleaner.com/
WinDirStat  for figuring out how I managed to fill up that 1TB drive and how to get the most space back with the least amount of effort. http://windirstat.info/
OpenOffice.org : The Free and Open Productivity Suite
Doesn't matter what some say, I like it better than MS Office, as it fulfills my needs and doesn't have as much tendency to corrupt complex documents. http://www.openoffice.org/
It is much faster than VLC if you are watching high quality movies, will play some files just fine which lag in VLC. http://sourceforge.net/projects/guliverkli/
Vim  (the best modal editor in the world) http://www.vim.org/
Gmail  https://mail.google.com/mail/
Dropbox  for syncing with other computers (Win/Mac/Linux). http://www.getdropbox.com/
On Microsoft Windows, for image viewing, IrfanView . http://www.irfanview.com/
ImgBurn  The best free ISO burner. http://www.imgburn.com/
WinMerge is an Open Source differencing and merging tool for Windows. WinMerge can compare both folders and files, presenting differences in a visual text format that is easy to understand and handle.
VNC  in all  of its  variants . Nothing says "sharing" like a program that lets you remotely control your work desktop from home...along with all of the servers...and workstations...handheld devices too... http://www.tightvnc.com/
Thunderbird  http://www.mozillamessaging.com/en-US/thunderbird/
. It's no Nero, but it's the best free CD/DVD burner out there.
It's for Windows, gratis, not open.
Eclipse  (a must have for Computer Science students, and others alike) http://www.eclipse.org/
Opera web browser . http://www.opera.com/
foobar2000  light, fast, awesome music player. http://www.foobar2000.org/
ClipX  stores a buffer of copy operations. http://bluemars.org/clipx/
PowerToys for XP  - Surprised not to see them on this list already. I can't live without Command Prompt Here, and the Image Resizer, ClearType Tuner and a couple of the other toys are great aswell. http://www.microsoft.com/windowsxp/downloads/powertoys/xppowertoys.mspx
Screen  http://www.gnu.org/software/screen/
Emacs  (the best editor in the world) http://www.gnu.org/software/emacs/
GIMP  - The GNU Image Manipulation Program http://www.gimp.org/
VirtualBox  for Virtual Machines. http://www.virtualbox.org/
subversion  http://subversion.tigris.org
THE most important FireFox plugin for me:
NoScript  http://noscript.net/
Ubuntu and everything that comes with it.
Git  http://git-scm.com/
Skype  because no-one needs to pay for international calls anymore. http://www.skype.com/intl/en-gb/
Blender , a free open source 3D content creation suite. http://www.blender.org
Netbeans  (a must have for Computer Science students, and others alike) http://www.netbeans.org/
Google Earth ... fantastic piece of software and a great community. http://earth.google.com/
Unlocker  : Very usefull when you want to move or delete a file open by a process. http://ccollomb.free.fr/unlocker/
FLAC  - Free Lossless Audio Codec http://flac.sourceforge.net/
Sumatra PDF  - A PDF Viewer for Windows http://blog.kowalczyk.info/software/sumatrapdf/index.html
For Launchy fans, under Linux in Gnome there is gnome-do . http://do.davebsd.com/index.shtml
InfraRecorder is a free CD/DVD burning solution for Microsoft Windows. It offers a wide range of powerful features; all through an easy to use application interface and Windows Explorer integration.
InfraRecorder is released under GPL version 3.
Rockbox is an open source firmware for mp3 players, written from scratch. It runs on a wide range of players:
- Apple: 1st through 5.5th generation iPod, iPod Mini and 1st generation iPod Nano (not the Shuffle, 2nd/3rd/4th gen Nano, Classic or Touch)
- Archos: Jukebox 5000, 6000, Studio, Recorder, FM Recorder, Recorder V2 and Ondio
- Cowon: iAudio X5, X5V, X5L, M5, M5L, M3 and M3L
- iriver: iHP100 series, H100 series, H300 series and H10 series
- Olympus: M:Robe 100
- SanDisk: Sansa c200 series, e200 series and e200R series (not the AMS models)
- Toshiba: Gigabeat X and F series (not the S series)
Logmein.com  remote login web-base software (it's Java) that works through corporate firewalls and is free.
Update: It will also work with just a Firefox plugin https://secure.logmein.com/products/free/register.asp
Windows Live Writer  - For writing blog posts. http://windowslivewriter.spaces.live.com/
Google Reader  http://www.google.com/reader/view/
Jing  - Great little screencasting tool. It makes taking screenshots/videos really simple, and allowing some inline editing capabilities to windows. Plus it makes it simple to upload the screencasts to a web application or save them locally, whatever you want. Here's an example screenshot  and video  I just made while writing this post. http://www.jingproject.com/
Textpad is excellent and the free version is great. I've used it for over a decade now.
Total Commander  http://www.ghisler.com/
mplayer  http://www.mplayerhq.hu
VirtualDub , Video Editing Software. I use it a lot with Fraps for my gameplay videos. http://virtualdub.org/
The Combined Community Codec Pack  for video playback, best option I've found in terms of compatibility with various media formats. (decent bit better than just MPC:HC) http://www.cccp-project.net/
Exact Audio Copy  - CD ripper. http://www.exactaudiocopy.de/
For the Mac
Perian  Codec Pack with QuickTime Components http://perian.org
Old Space Monger  - Graphically display the disk space on your computer. The old one (Version 1.4) is completely free. The newer 2.0 one is not free. http://www.sixty-five.cc/sm/v1x.php
 - paste w/o formatting. Windows hot-key (default is
Windows+V) that allows you to paste text to any application without formatting.
So for the number n of window switches you need to get to the correct application window, it enables you to simply do
Ctrl+C, n ×
Windows-V instead of needing to jump through the complete
Alt-F4, n ×
Ctrl+V hoop. ;-)
speedcrunch  is a nice replacement for the default calculator http://speedcrunch.org/
BatteryBar . I install this on all my laptops and all my family and friends' laptops. It's a great battery meter for Windows with an extremely accurate time remaining prediction.
(Full disclosure: I'm the author, but the program is free) http://osirisdevelopment.com/BatteryBar/index.html
FireFtp for Firefox!
FeedDemon  - Windows desktop RSS feed reader http://www.newsgator.com/Individuals/FeedDemon/Default.aspx
POV-Ray , a fast and free raytracer, with an intriguing SDL. http://www.povray.org
Cygwin ! : ) A unix terminal and utilities that can be run on windows. http://www.cygwin.com/
Remote Desktop  http://support.microsoft.com/kb/925876
Foxit PDF Reader  much much faster than Acrobat. http://www.foxitsoftware.com/pdf/reader/
Teamviewer  : the most simple way to control another computer.
Very usefull to solve friend problem or if you need support! http://www.teamviewer.com
(www.voidtools.com) helps on my NTFS drives when I can't remember where I put something - much quicker and easier to list lengthy results than Launchy (also good)
Microsoft Security Essentials - light, simple, and easy to set up.
Notepad2  http://www.flos-freeware.ch/notepad2.html
Rapid Environment Editor is a nice tool for editing environment variables in Windows. http://www.rapidee.com/en/about
Microsoft's Live Mesh  for sharing files between PCs. This wasn't a "must have" until I started using it, now I can't live without it!
UPDATED: Live Mesh has been replaced by Windows Live Sync . https://www.mesh.com
CDex  - For ripping CDs to MP3s http://cdexos.sourceforge.net/
Songbird  - Free music player with addons, skins ("feathers"), etc. Built on the same technology that Firefox and Thunderbird use. Can sync with your iPod, iLike, Last.fm, SHOUTcast etc.
Nautilus  (with thumbnails of RAW files, it makes photography easy) http://live.gnome.org/Nautilus
Belarc Advisor  is a nifty tool to get an inventory of installed everything and even the registration keys. It has come in quite handy upon a rebuild of my machines. What I do is run this prior to reformating, save it in the archive and refer to it as I rebuild the machine. Most specifically I get my Windows Vista key back without going to look for it and can copy/paste it during the install. Not to mention referring to the inventory of software I had installed prior.. http://www.belarc.com/free%5Fdownload.html
FastStone Image Viewer : viewing/slidshowing/quick editing images in Windows. http://www.faststone.org/
Avast antivirus . Home version is free and quite effective. http://www.avast.com/
Comodo firewall : free but one of the most bulletproof solutions. http://personalfirewall.comodo.com/
Imagemagick , a free image conversion tool, useful to perform batch conversion of images in a variety of formats. http://www.imagemagick.org
XAMPP  - a powerfull, fast, free, and easy to set up LAMP environment. Great for playing around with Drupal. http://www.apachefriends.org/en/xampp.html
Trillian  - IM client that supports AOL/MSNM/Yahoo/ICQ/Bonjour/FaceBook/Jabber/Skype/Twitter... http://www.trillian.im/
Cute PDF Writer  a printer driver to generate PDFs. http://www.cutepdf.com/products/cutepdf/Writer.asp
iTunes  for Mac and Windows. http://www.apple.com/itunes/
Infrarecorder . The best CD/DVD/IMG/ISO burning program I've ever used. http://infrarecorder.org/
PSPad  editor is a freeware text editor and source editor intended for use by programmers.
PSPad has many software development-oriented features, such as syntax highlighting and hex editing, and is designed as a universal GUI for editing many languages including PHP, Perl, HTML, and Java. It integrates the use of many project formats for handling and saving multiple files. Other features include autocompletion, tabs, FTP client and find/replace using regular expressions.
Enso  - I love this launcher much more than launchy. I find it much more natural to use than Launchy/Gnome-Do. Plus it finally puts Caps Lock to good use. http://www.humanized.com/enso/
Clonezilla for imaging
Whenever I am looking for a utility, I always search sourceforge to see if something comes up. Great site when you want an app, but don't have a specific one in mind, and don't want to pay for it.
ExamDiff  - simple, lightweight file comparison tool.
Technically, I guess it's freemium, because they also sell a "pro" version, but I've never had any complaints with the free version. http://www.prestosoft.com/edp%5Fexamdiff.asp
PrimoPDF  - Free PDF printer/writer
Small footprint and excellent compatibility. http://www.primopdf.com/
The Wonderful Icon  - tray app that allows extensive manipulation of program startup behaviour and, its pièce de résistance, allows you to minimise any program to the tray with a right-click. Runs as a service so you don't even see it once installed and configured. http://www.thewonderfulicon.com/
Simplify Media  - for sharing my MP3 file collection to my iPhone or other PCs http://www.simplifymedia.com/
TextWrangler  http://www.barebones.com/products/textwrangler/
WhatColor  - Tool for determining the RGB values of a pixel on your screen. http://www.hikarun.com/e/
Brasero Burning Software  (really easy and underrated) http://projects.gnome.org/brasero/
TUGZip . Free archiving utility with Windows integration, script support and ZIP-64 for huge archives. http://www.tugzip.com/
baregrep  - for grepping (searching) through files rapidly, with the ability to search for patterns matching regular expressions. great for searching through log files, documents, source code, etc. http://www.baremetalsoft.com/baregrep/
grepWin  Regular expression search and replace for Windows http://tools.tortoisesvn.net/grepWin
Infrarecorder  is a quick, small and convenient disc burning utility that I use on WindowsXP. http://infrarecorder.org/
Mount images just as easy as burning them!
Free, good support, decent recovery database making it possible to clean files after they've been infected unnoticed.
Moria , one of the best games ever... ;-) http://en.wikipedia.org/wiki/Moria%5F%28video%5Fgame%29
Happy Lad Image Mapper http://www.indiepages.com/diyhtml/index.html
Hijack This  http://free.antivirus.com/hijackthis/
Colibri  (http://colibri.leetspeak.org/) is a must have navigator/launcher on Windows - it remembers frequently used locations. http://colibri.leetspeak.org/
InfoZip  free DOS based zip / unzip utility (Works better than windows zip functions) http://www.info-zip.org/
Executor - similar to Launchy but way way more powerful - has customized keywords and internal commands, very easy to tweak and set up - also VERY LIGHT on system resources. a MUST HAVE!
WinPatrol - keeps computer running in lean form!
GOM Player 
Powerful subtitle support
Play Broken AVI Files
Easy Navigation of Video play with different speeds
Microsoft Security Essentials. It is doing a very good job for me.
Musescore Music Notation Program. Open source and great support. http://musescore.org/
IsoRecorder  is a great Explorer shell extension to enable burning and ripping of CD/DVD images. Between that and the integrated burning capabilities of Media Player and Explorer, that's most of the burning I ever need. IsoRecorder is clean and non-intrusive. http://isorecorder.alexfeinman.com/Vista.htm
Others have mentioned text editors...my preference is Text Wrangler . http://www.barebones.com/products/TextWrangler/
Apple's Safari . http://www.apple.com/safari/
LiberKey  - IMHO the best free portable/local app bundle you can find! It comes in 3 sizes, you can easily add any other supported app from their site list, it's auto-update is a bliss! And the best: it already contains most of the apps listed here! :)
Quote from their site: "LiberKey includes a lot of free software. True "swiss army knife", it offers local and/or portable use" http://www.liberkey.com/en/