Check out sfxr, an awesome awesome awesome video game noise generator.

I've seriously been sitting here for the last half hour just goofing around with it. I legally shouldn't be allowed to enjoy this so much. If only the real world sounded this beautiful.

Hm. I wonder how hard it would be to use this to generate noises/notes for some home-brewed chiptune-like beats... but probably nobody in the world would like listening to that stuff but me.

Update: Looks like the same person is working on an unbelievably cool looking music editor/synthesizer called musagi which seems extremely well suited for such means. I hope this gets released as free and open source software some day.

Another update: Sorry to keep spazzing about the subject, but you should seriously check out this dude's article on how sound works. As someone who's never studied the subject, I find myself surprised to be mostly understanding this. Really an illuminating read.