It depends on what you want to do.
For simple sound editing and ability to add some effects and manipulate a single sound, Audacity: Free Audio Editor and Recorder
(I know audacity can do multi-track stuff, but that's not it's strong point)
If you want to do multi-track recording like a sound studio, check out REAPER | About
If you search through their download archives, the pre-v1.0 downloads are totally free.
If you want looping, you can't beat Sony Creative Software - ACID Pro, ACID Music Studio, and American Idol Extreme Music Creator
It's not free. The pro version is expensive. But you can't beat it for looping.