I used to use Logic Pro, but my friend took back the USB key necessary to use it and so now I can't use it anymore. :\ Too bad, because it really was a great program. Now I use Finale 2006 (thanks nik!) to compose, as well as record input from my USB keyboard, and it works great, it just screws up the notation when I input from a keyboard. I didn't realize it still stored it as I played it until later. I tried FL Studio and it's interface is nasty. I didn't like it at all.

Oh, and who could forget Csound? My god that program can do exponentially more than any other sound software program in existence. It is the best in terms of functionality, open source, versatility, and flexibility, but oh man the learning curve is steep as hell.