The blog about painting with milk reminded me of this.

This is something I did as a creative exercise for people at my work. I gave everyone 5 bottle caps and some paint and told them to paint whatever they wanted on them. Then I made a wood frame to hold them. I made it so it would have an interesting wave pattern. When everyone was done, we stuck them on and made a cool mosaic. The cool thing is, 90 percent of the people who contributed to this aren't artists (I mean they are, but they just don't know they are), yet it still made something cool and beautiful to look at. We still have to make more to fill it up, but it actually even looks cool with the bare wood spots.