Here is one amazing audio tool, I managed to create a crappy song with it, and it is similar to FruityLoops, but this one is free:
http://burn-studios.audiotool.com/
An audio mixing tool, not quite as feature packed as Audiotool, but has a lot of samples built in.
http://www.looplabs.com/
Audacity is now a multi-track audio editor, and it can load MP3s to mix songs on different tracks. I've used it before and it's great:
http://audacity.sourceforge.net/