I thought it would be a good idea to start a thread of all of the useful free and open source software out there. For reference to other people.
Do not list Untangle please, that one is pretty obvious.
Things I recommend;
7-zip - Archiving utility for Windows, it can open tar.gz files in Windows
Hash Tab - Checksum utility for Windows
SubSonic - Music streaming software
ClamWin - Windows port of the Clam anti-virus
Will add more as I think of them, please everyone add your .