While you can't beat the price of Audacity, and it does have the ability to multi-track, I think Sound Studio is the best recording software under $100 for the Mac. ($79.99) Intuitive and stable, with a lot of bang for the buck; effects processing, loops, etc. Also check out Transcribe, a program that can change speed or pitch, add karaoke effects, various other enhancements, for $50.