Music Animation Machien (shortened to "MAM") is a MIDI player, but it's more than that. MIDI's might be considered to be sound files, but MAM definitely makes them something more. In addition to being able to play MIDI files, it also has a visualizer, and a pretty sweet one at that. It has 12 different views you can choose, differing from the regular piano roll-esque dancing lines to views that really have music theory behind them. Either way, no matter what you choose, you can most certainly say "Pretty colors" to any of them. MAM takes a midi file and turns it into a musical screensaver.
You can change colors, show the note start lines, and choose whether the notes are colored due to pitch or by what track it is. If you choose pitch, you can set the tonic, thus changing the colors of the notes. You can also view the track by right clicking in certain views.
MAM also has a limited ability to accept MIDI input as well. For example, it has the ability for "Live play", meaning that it will create the visualization as you play on your MIDI controller. Let me be clear, though, in saying that MAM is not a MIDI editor. You can save files, but all it does is save the color preferences for later usages of MAM.
It's the niftiest MIDI player I've yet seen. The only thing that I'm kind of said about is that it doesn't have a fullscreen option. If it did, you could rig it to be your screensaver, opening a random MIDI and taking up the full screen. But alas. Anyway, one of the other things about MAM that is great is that it is very tiny, weighing in less than 1MB, and is portable.
It's really nifty. There are a ton of videos of neat works that are just played through MAM. It's fun to search around and find works you like and then toy with the color and view settings until you get a really great lightshow.