Everyone is probably aware that Google can search not only text but also videos and photos. For example, if you right-click on a photo on a web page, you can find out where else on the web that photo is available. The image finder is available in the standard interface or at images.google.com.
However, there is a hidden feature that few people know about: the search can even search for an image stored offline, simply by locating the photo on your hard drive and dragging it into the search box. Google can also search for a specific image, but it can also find similar photos for you.
A similar solution is offered by a service called Tineye, available at tineye.com.