What's the closest equivalent to Sonic Foundry's Acid Pro for the Mac? It's what everyone uses to make mash-ups -- acapella vocal over cut-up music and loops, with pitch-shifting and time-stretching -- and I want to be able to do this easily on a Mac. Would Ableton Live work? Is it the best alternative? HELP!

Reason + ReCycle, or perhaps Live.

You could get Garage Band or Sound Tracks, I have Sound Tracks, it came with FCP4. I use Acid loops in it and have recorded my own loops, it has a loop utility for putting in meta data. Reason is fun to use I think it sounds a little thin, it is not a loop editor however.

Apple hired the software developers of Acid, the result was Soundtracks, and some of the features of Gararge Band.

Soundtracks can load Acid format files.

Apple Loops is like Acid on steroids, they can be audio only or contain midi info also.

for the Apple Loop Utility for making Apple loops download the Apple Loops SDK.