I just spent the better part of two days trying to remember what the hell they called those things.

Del Sol

Scroll down toward the middle of the page and you can find links to a few of the t-shirts that change. My sister bought one of these a few years ago. They really are cool. They have taken it to another level now, though. I've seen fingernail polish that changes color from the sunlight. I can't find a link at the moment. Still looking for that...