If you absolutely can't spend money on anything, I'd go with PxTone for bare-bones prototyping. It runs on Wine. It is point-and-click tracker.

http://buzinkai.net/PXTone/tutorial/

It's an incredibly lightweight tracker or whatever, and comes with preloaded instruments (basic chiptune-ish sounds). But when you're prototyping a song or trying to get an idea down really fast, you shouldn't worry too much about the instruments or anything. PxTone is strong in this regard, it's VERY FAST and intuitive to just jot down a melody.

I find PxTone is great if I think of some short melody and want it sort of stored so I can come back to it later and maybe work with it on something more involved than PxTone.

Of course it's mostly useful for the prototyping stage. Any sort of recording with real instruments, or effects more than just panning/volume/reverb/echo/overdrive/portamento, you'll want to look elsewhere.