If you're a loop freak and you work on a Mac, Girl ($99) from Yowstar (http://girl.yowstar.com) should get your creative juices flowing. Girl can slice, dice, repitch, granulate, filter, delay, loop, and resequence your sound files in real time. The number of simultaneous files it can handle is limited only by your CPU and RAM.
Most of Girl's action takes place in the Mod window, where you select the file you want to process and set up the processing options. These include classic-style pitch shifting (in which speed changes with pitch), pan and volume LFOs, and amplitude envelopes. Next comes a parallel pair of multimode filters, each with its own frequency, resonance, and gain LFOs. The filters are followed by a feedback delay line, again with a dedicated delay-time LFO. However, things start to become really interesting when you get to the grain and sequencer sections.
The grain processor is basic, offering control over grain size, grain-sequencing rate, and random grain shifting. The step sequencer is an eight-step slice sequencer. You set the slice size by setting the number of slices into which the sound file will be cut. The sequencer will play the slices in any order and at any tempo. Combining that with the grain processor can mangle your loop beyond recognition. There are no provisions for MIDI control of Girl's settings, but most settings have computer-keyboard shortcuts.
Girl will record the output of any individual module or a stereo mix of the combined output of all modules. There is also a provision for syncing the different modules to one tempo, which makes it easy to combine and process several loops into a single loop. The MP3 file AtaGirl is a simple drum 'n' bass loop taken through several phases of processing. Yowstar does not specify minimum requirements, but Girl ran well given 64 MB of RAM on my G3/300 MHz with Mac OS 8.6.