Recording music (I play bass guitar and a bit of electric guitar, but I haven't tried recording them)
It really depends on what you want to do. If you just want to have the sounds to listen to, then Audacity is the way to go. If you want to do more than that, look at some DAWs (Digital Audio Workstations) like Reaper, Logic (Mac Only), Ableton, and Fruity Loops, but keep in mind that they're often expensive. However, if you're wanting something legally free, take a look at LMMS (it has Linux in the name, but runs on everything).