One site is "www.ffffound.com". Start with a picture you like and "FFFFOUND" will show you more pictures marked by different members that you could like. Really interesting if you´re a member but also just to browse through. They say: "FFFFOUND! is a web service that not only allows the users to post and share their favorite images found on the web, but also dynamically recommends each user's tastes and interests for an inspirational image-bookmarking experience!!" Right now the site is still Beta and it´s not possible to register.
Hopefully that will change soon.

"Retrievr" is very interesting, too. Find Pictures by drawing or uploading pictures. Sometimes the results are quite good sometimes they are not but they are still working on that project and it´s already fun using it. The Developers say: "Retrievr lets you find flickr images by drawing rough sketches of them. Finding images on Flickr is mostly textual (tags, keywords) or social (contacts, friends, groups). Retrievr is, like images, visual. At the same time it's our testbed for image retrieval algorithms, so that when you add an image to a page in System One, it gets you the potentially most similar pictures back in realtime."

Another way is finding pictures by choosing their main color. Try the "Flickr Color Selectr".