It's not free, but in my opinion, well worth the price. . . Logic.
I fooled around with audacity, but just never liked it. Moved to SoundStudio when I was a 2-channel guy. Tried Soundtrack, which I already owned when I moved to 4-channels, but found it to be less than elegant for what I needed to do. Picked up Logic Express (the scaled down version of Logic) because it was much cheaper. Rolled with that for a few years, then used the upgrade path to the PRo version when v 9 was realeased.
Intuitive interface, nice set of included plugin's. Easy to use.
-j