Here’s a handy site: TinEye. It’s a reverse image search.
If you’ve got an image on your machine and don’t know where it came from, or you find it online and wonder where the original lives or who uploaded it first, this can help.
I learned about this from Terence Eden, who has a great example on his blog showing how it can be useful…
It seems to be more comprehensive than google’s reverse image search.