Ableton Live is excellent. Version 6 even allows you to sample video and put it in time with your musical creations.
You can use it to sample, create midi sounds, add literally hundreds of effects to your sounds, use wavs/mp3s etc
The nice part about it is you record live, like jam sessions, or bursts of creativity and then you can integrate it into pretty much anything, hook the program up to your decks and drop pretty much any sound into your mixes in time, mash up beats that are playing etc etc etc
very powerful program:
http://www.ableton.com/