From TDN

Audacity - Free, open source multitrack audio editor. Easy to use and very powerful. Available for Windows, Linux and OS X.

Adobe Audition - Previously Cool Edit Pro. Destructive, non-destructive and multitrack editing with effects.

Protools LE - Hardware I/O (Mbox) purchase required. Industry standard non-destructive multitrack audio processing for effects, music, and video.

Reason - Extremely intuitive midi synth program which links to sequencer for music creation. Can use Rewire to link to other audio programs.

Ableton Live - DJ sequencer turned improv music generator. Extremely powerful and flexible to create music and ambient scapes.