Chrome only: Google's Similar Pages for Chrome extension adds a button to the toolbar that uses Google's search algorithms to find other websites similar to the one you are currently browsing.
Once you've installed the extension, you can simply click the new button in the toolbar to see a list of related pages, complete with thumbnail previews, descriptions and a link. The related sites are usually fairly accurate, but where it seems to work best is when browsing more permanent articles on a specific subject — for example, using the similar pages button on the Wikipedia article on Shadow Copy brought back a list of useful related pages, but if you are on the home page of a site, the results will be more general.
If you need to do a lot of research and can't always think of the right query to use on Google, this extension could come in handy. Google Similar Pages is a free download, works wherever Chrome extensions do.