Top 10 open source Windows applications

1. Mozilla Firefox (Web browser)
Fast and secure internet browser
http://mozilla.org/firefox/

2. Mozilla Thunderbird (Email client)
Mozilla’s Thunderbird 2 email application is more powerful than ever. It’s now even easier to organize, secure and customize your mail.
http://mozilla.org/thunderbird/

3. Open Office (Office suite)
OpenOffice.org is a multiplatform and multilingual office suite and an open-source project. Compatible with all other major office suites, the product is free to download, use, and distribute.
http://www.openoffice.org/

4. Gaim (Instant messenger)
Chat on any service you'd like - AIM, Yahoo! Messenger, MSN, Jabber, ICQ - with this multi-platform, tabbed IM client.
http://gaim.sourceforge.net/

5. ClamWin (Antivirus)
Norton bugging you again to break out the credit card and subscribe? Uninstall! ClamWin is free anti-virus software with automatic updates and scheduled scans, no credit card required.
http://www.clamwin.com/

6. VLC Media Player (Audio/video player)
Got a video or audio file Windows Media Player or Quicktime can't play? Betcha VLC can.
http://www.videolan.org/vlc/

7. KeePass (Password manager)
Another app you really don't hear a lot about, but for anyone with more than 6 different passwords, KeePass is indispensable.
http://keepass.sourceforge.net/

8. Cygwin (Unix command line emulator)

That DOS command line just doesn't cut it. Wanna turn into a CLI ninja on your PC? You need Cygwin.
http://cygwin.com/

9. Eraser (Data deletion utility)
Before you donate, sell or trash your hard drive, you want to make sure there are no traces of your private data on it. Eraser uses the same algorithm the government uses to wipe your hard drive clean.
http://www.tolvanen.com/eraser/

10. TrueCrypt (File encryption utility)
You've got a folder full of files you don't want anyone to access but you. Lock it (or an entire thumb drive) up with the free TrueCrypt software.
http://www.truecrypt.org/

Some references:
http://lifehacker.com/
www.clamwin.com/
http://softlabsav.softlabs.net/