MacJams.com, the music community for amateur and aspiring musicians, has released the 4th batch of free loops in their 5-week set. I posted this over on DownloadSquad, but I thought it was too cool to not mention here on TUAW. The loops are in Apple Loops format for the likes of GarageBand and (I think) Final Cut/Soundtrack. I've been too busy with a new semester to crack them open, but apparently they're just regular AIFF files any audio editing program should be able to use. In fact, one DLS reader was using in Sony's ACID Express on *shudder* Windows. I've also heard the loops touch on a range of sounds like ambient, hip-hop and jazz, so they sound pretty handy.
Go check the series out, especially since you have no excuse not to - they're free! They're also licensed for non-commercial use, and their site has more information on all that legalese. Enjoy!