I just bought Live 4.1 from Ableton. I love it. It's the perfect music production software for DJ types like me. You can import sounds, midi files, effects and fine tune the loops and samples. The neat thing is that you then bind loops, tracks, effects and other things to keys or midi events and jam away live to your heart's delight laying down a recording that you can then go back and edit before you render it. It's a bit hard to explain. Take a look at the demo on their site.
Here's my first track using samples from Lessig, Jimmy Wales, Kenji Eno, Howard and others. ;-) (mixup1.mp3 1.9 MB mp3). It's a bit rough, but you get the idea.