If you've never used Acid, yer missing out on a lot M... (and I don't mean the alligators under the couch type of acid).
Acid is a visual editing tool for audio -- you drop tracks, loops, vocals, whatever, on a time line, and can easily make envelopes (sections of audio), and change any aspect of it.
I use it to create songs, then overdub (punch in) my instrument parts on top of the loops.
Here's an example of an Acid thing I did 10 years ago. I did almost all of it with loop samples, and overlaid some keyboards I did, and then some samples I took from Star Trek...
This is called 1709 Point 2. I did it in a couple hours, and was quite buzzed at the time in the middle of the night, after coming home from a club...
http://www.softjock.com/mp3/1709-CptRick.mp3