Below are my recommendations for free or open-source applications in a variety of categories, from software development to data security to simple utilities. I use all of these apps personally, either at work, at home, or both, and consider these to be the best among the free alternatives that I've tried. As you can see, many of these are available for both Windows and Linux, which in many cases is one of the reasons I like the app, since it allows me to use a familiar app in multiple environments.

Category	Application Name	Windows	Linux	Mac
.NET Analysis	Reflector	X
.NET Build	NAnt	X
.NET IDE	Microsoft Visual Studio Express Editions	X
.NET Unit Testing	NUnit	X
Anti-Virus	avast!	X	X
Continuous Integration	CruiseControl	X
Database	Microsoft SQL Server Express	X
E-Mail Client	Mozilla Thunderbird	X	X	X
Encryption	TrueCrypt	X	X	X
File Compression	7-Zip	X
FTP Client	FileZilla	X	X	X
Hard Disk Analysis	DiskBench	X
Hex Editor	HEdit	X
Icon Creator	Imagicon	X
Instant Messenger	Pidgin	X	X	X
Media Player	VLC Media Player	X	X	X
Office	OpenOffice.org	X	X	X
Operating System	Ubuntu (GNU/Linux)
PDF Viewer	Foxit Reader	X	X
PDF Writer	CutePDF Writer	X
Photo Editor	GIMP	X	X	X
Sound Editor	Audacity	X	X	X
Text Editor	Notepad++	X
Textfile Change Watcher	Tail for Win32	X
Twitter Client	TweetDeck	X	X	X
Virtualization	Sun VirtualBox	X	X	X
VNC	RealVNC	X	X
Web Browser	Mozilla Firefox	X	X	X