SooperLooper is a live looping sampler capable of immediate loop recording, overdubbing, multiplying, reversing and more.
The application is a standalone JACK client with an engine controllable via OSC and MIDI. It also includes a GUI which communicates with the engine via OSC (even over a network) for user-friendly control on a desktop. However, this kind of live performance looping tool is most effectively used via hardware (midi footpedals, etc) and the engine can be run standalone on a computer without a monitor.
SooperLooper is currently supported on Linux and Mac OS X, and any other platforms that support JACK. The Mac OS X package is usable with at least the 0.6 release of JACK OS X. Note that for OS X Tiger, you'll need to get at least version 0.7 of JACK-OSX.