similar-site. is one of the new class of search engines that will find sites similar to one that you already know. Just type in an appropriate URL and it will go off and find new sites for you to view. This one provides a percentage figure of confidence. I played around with a few urls - my site, my weblog, BBC site, CILIP and so on. I was fairly impressed with the results that I got, and didn't find any howling errors - on the contrary I was getting really good stuff.
The interface is very basic and may well be offputting to some, but if it's quick results that you're after, this engine will do very well indeed.
[Edited to add: If you want more engines of this type, I've got a list (Let's face it, when it comes to search engines, I've always got a list!) of several at: http://www.philb.com/sitebasedinfo.htm#similar)