I use Cool Edit Pro, which is the precursor to Adobe Audition. It takes a little getting used to because you right-click-and-drag to move clips, whereas most others use left-click-and-drag. If you can afford Audition, I recommend it.
Soundbooth is okay -- I have CS5 for work, and that's got multitrack, but it's not as good as Audition.
For a free option, Audacity is pretty great.