I use music from incompetech.com. Its written and performed by Kevin MacLeod and he makes it available royalty-fee, in other words as long as he's given proper credit (I always mention his name in the outro and his name, the song used, and website appear in the credits section of my readme) then you can use it without paying, though he does accept and appreciate paypal donations.
Fox, I utterly and seriously love you right now - I've been looking for something like this. Now, here's the question I have - which piece do you usually use? Because I don't want to pick something you've already had...
M@