A few days ago I discovered an amazing music program hosted on the game site Kongregate. Apparently the creator, the guy who runs QuickFingers.net is in some sort of contest for a LOT of money to see who can make the coolest music game on that site. Well, his was on top when I checked a few days ago, and I tried it out. It was AWESOME!! The game he created is called Step Seq (try it at https://www.kongregate.com/games/quickfingerz/step-seq).
Really, this is just like any hardware-based step sequencer, which is apparently some sort of grid of buttons that you can press and it will modulate a midi device and play back notes. Each row on the grid is a single note, while each column is some sort of time index. The sequencer part is based on a concept that the grid will play itself over and over and you can change the settings each time.