Mario Paint Composer
Mario Paint Composer is originally built into the game Mario Paint for Super Nintendo, but has recently been ported to Windows and Mac.
This game was groundbreaking, and when I was a kid, I just had to have it. It came with a plastic mouse and plastic mousepad, unlike any other game. The game allowed you to draw and to compose music by selecting Mario sprites. Each Mario themed sprite being a different instrument, and you just copy these onto the music staff, and play the song you created. This was amazing at the time when people didn't have computers that can do this same thing. Plus, it had Mario sounds, so the music was fun.
Today it has a following on the Internet for the unique 90s era sound that only this program can produce.