I've recently made available a little project of mine called LoopCenter. It
is a live-looping tool with a functionality very similar to the Boss
LoopStation pedal. I use it mainly as a improvisation practice tool, but it
could certainly be used for more compositionally-oriented stuff.
Basically, you can set a tempo, and easily record and overdub small,
measurized segments of audio, and then loop them. It is similar to
SooperLooper (an excellent piece of software that I found only after making
LoopCenter!), but pared-down, and perhaps easier to use. It runs with JACK.