SketchSwap: It's All About Give and Take

What a well designed system!
In my travels in del.icio.us, the social bookmarking site, I came
across a site that's beautiful in its simplicity. It's called SketchSwap.
You sketch something and submit it.
In return, you get a sketch back that someone else drew.
No one gets a grade.
No one keeps score.
You give something and you get something.
Nice, huh?