Mark is spot on about earlier versions of Ableton's Live being as closely analagous to Acid as anything you'll find for Os 9 or X, for that matter. As much in love with ACID as I am, Ableton's is rapidly outstripping it as a program, especially with there new midi sequencing ability and VST support. I don't have the bucks for the upgrade yet, but I look forward to getting 4.0. There are several artists on Loopers Delight who have it by now, prominently among them, Per Boysen, I imagine. I think Cycling 74's RADIAL is a really interesting program. Invented by musical rennaisance programmer/recording artist JHNO it uses a multiple simultaneous looping paradigm that allows for some interesting justtapositions of loops that go at different speeds and cycle lengths. It is so different from Acid, however , that I don't think it is a good analagous solution for Acid. Acid is really more like a cross between a looping program and an audio program (like the audio recording parts of Cubase, Sonar, Logic, Digital Performer, et. al.). I think Ableton is your best bet.