Staffan recently tipped me about the Beaterator, a Rockstar Games virtual drum machine. Using it, you tweak old skool drum-machine beats with loops from the likes of Juan Atkins and others.

If you register, which is free, you get to save your work, your loops and can download your stuff as MP3's. Very cool! [the backside being you're added to the Rockstar Games mailing list, which isn't as bad as it seems; I've been on it for ages and get mail appx. three times a year]