Can't see this talked about elsewhere but I've been trying to recall this site/plugin for ages and suddenly it popped up in a lifehacker post!

http://www.tineye.com/

Either install as a plugin, use from windows shell or go to the site & select an image or link to the URL to find 'similar' images on the web.

Obviously it can get confused with similar piccies (ie one of mine of the Forth Rail bridge comes up with a couple of 'hits') and it isn't perfect but I think some people will find it useful especially on this forum!

Right, I'm off to search every image in the Photo-a-Day thread...

Rob