The complains of other users here are probably valid, but I for one have been having a really great time with the demo of Traktor DJ Studio. It runs for an hour before quitting, plenty of time to learn how it works. And its feature set is very good. It does automatic tempo matching and beat detection; you can create multiple beat-precise loops within tracks -- on-the-fly even; you can set markers to indicate where within this track the next one should automatically start playing.... Anyone who thinks they want to be a DJ should give this a try. There's no extra hardware to buy! All you need is your Mac and a library of MP3s. (Keeping my fingers crossed for AAC support soon....) This program is so much fun.