If you've given your students an assignment that involves images, how do you know if they created it themselves, or if they copied it from the web? Now there's a way to find out.
TinEye.com is search engine with this purpose in mind. Give it an image, and it will tell you where else that image is on the web, and sometimes, where the original image came from.
Ars Technica has given a thorough review of TinEye that is worth the read.
I tried finding the source of the Einstein picture I use for my Teaching with Blackboard course so I could credit it, but couldn't track it down from the 838 matches TinEye found of it.