I've been searching a long time for something like this. It's not on OSX, and it's not an audiounit plugin. It's called ProLoop and I got it for $5 on the iTunes store. It's a very hands-on loop player capable of handling 6 loops at a time. My usual audio clips didn't feel like it was fully explaining what's going on, so be sure to check out the video above for a slightly more in-depth look.
So this thing plays up to 6 loops at a time. Each loop can be pitchshifted up or down, and the user can select whether the pitchshift is locked / quantized to tones or not. I prefer smooth pitchshift, kinda like a digital tape varispeed. Each loop has forward or reverse, but it can also freeze in whatever position you're holding down on the screen. All of this is already enough for me, but they add a bonus: there's FM between the loops! Loop 1 can modulate 2 can modulate 3, etc. etc. Plus the user gets to choose between FM, PM or Ring modulation. Sweet.
The app comes with a bunch of its own loops, but I didn't use them in this post. I used some existing audio from previous posts on here. Currently, there's no way of trimming the loop size in the app, but no big deal, I just prepped them beforehand. Uploading my own loops to this app was super easy: all I had to do was type in some http address using Safari and I could upload directly from a 10.6.2 macbook. (The upload did not work from my 10.5.8 mac pro however.)
Be sure to check this one out, because I've barely scratched the surface here. This looks like a wonderful tool for creating some crazy sounds. My Soundcloud clips are probably gonna sound a bit more creative than what I came up with in the video. For the video I had to deal with sweaty hands, a tripod in my way, and trying not to bump into everything. Not too shabby for an iPhone that's been dropped into the toilet.