Audacity is free, open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems.
In the Frets On Fire community, Audacity is typically used to convert .mp3 files to .ogg files, though it can do much more. It has the ability to convert .wav, .ogg, and .mp3. However to convert a .wav or .ogg to mp3 you must have a separate program called LAME on your computer. Audacity does this to stear clear of legal trouble. LAME is useful if you want to put the songs onto an mp3 player from FoF, but .wav files will go onto most mp3 players (they're just really big).