This is similar to that other site where you use the white dots to make music except I like this one better.
You have a grid and you just draw different size squares/rectangles to make your loop. I found it really entertaining.
http://www.glitchscape.comShare your loops and stuff.
One of my loopsDo this for posting your loops:
[ url=YourLoopURL]Text[/url]
Without the space of course.