Zaen found this site and it looks pretty neat. It effectively lets you generate your own rhythm as a picture.
Site:
http://www.sembeo.com/media/Matrix.swf
Sample picture with .MP3 file provided.
I remember a site like this from a long time back it wasn't such a big grid. I think I may still have an image I for that one kicking around. In the meantime here's one I did for this.
Oh lord! When I first saw this on Tumblr, I freaked! <3 So glad you guys found out about this! ^^ I could do one myself, but I'm sure I'm not as clever as any of you. c':
It's all in the same key, so you actually can't really screw up. I wrote "Zelda" with my mouse and it still sounded okay :>
Facebooking this right fucking now.