TinEye is a slick "reverse lookup" tool that helps you find similar images from a source. It's the kind of thing that would be handy to be able to access from your browser's context menu: see an image you want to search, right click it, and get your results!
Firefox users have been able to add this functionality for a while using an addon, and now Google Chrome users have an option as well. A new extension adds a TinEye-powered 'search similar' entry to your Chrome context menu. You can see it popped up on my glorious foreground image of Earthworm Jim -- and TinEye's bevy of similar images peeking out around the edge.
Download: Chrome Extension