This is the world's largest repository of open source projects.  For anyone who's not familiar with the phrase "open source", basically it's software that's being developed (usually through a collaboration of multiple people) and made available to the public for free.  You have access to the source code so that you can make any changes, should you have the skill and inclination.

SourceForge is a gold mine!  There are over a 100,000 different open source projects.  If you are looking for a program to fufill a particular need, check here first to see if a free one is available. 

There's a heavy emphasis on programs for the Linux platform, since Linux is an open source operating system.  Having said that, there are a lot of programs available in both Windows and Linux versions.  There are even some Windows-only programs such as "Fluffy the SMTPGuardDog".

You can check out SourceForge at http://www.sourceforge.net.