When you think about it, music is all around us. That's usually because we have our iPods in our ears. Or our MP3s blasting through our computer. Or some concert DVD on the telly. Or we have the radio on. The point is, now you can play games and actually make music at the same time, isn't that something!
Mario Paint Composer (Mac/Win, 8MB, free) - With big cute graphics, a simple interface and the ability to do (well, almost) anything, Mario Paint Composer is a wonderful remake for the budding musician in all of us. Mix together the over dozen included sounds through any tempo and pitch to create that masterpiece cat-meow solo you've always dreamed of.
Mono World (Windows, 100kb, free) - In this music-based tool, you are given free reign in several different situations. Let your creativity roam as you figure out, on the fly, how to create wonderful melodies with sliding rods, laser beams or shaking spheres. Press escape for colour and enter to return to the menu. (Download link appears at the bottom of the developers page.)
Balldroppings (Mac/Win, 500kb-1MB, free) - Ever thought how, when you drop something, it should be more musical? Well grab a hold of Balldroppings and create a line or two dozen to bounce pixels off, up, and all over the screen. Each line plays a note as the ball, and subsequent over-lapping balls cascade around your screen. (Also available for some mobile devices for roughly $10)
The Crowd (mirror) (Windows, 7.6MB, free) - Although not a tool in which to make music, or even a game to play, The Crowd is an interesting and atmospheric experiment in behavior. Assign people either love, hate or security and see how they interact with others around them. Be sure to have your speakers on to experience the full, truly warped experience.