Flickr

I'm a big fan of Flickr. For me it's as much a search engine for photos as it is about photo sharing. So tagging images with good keyword tags is important. But I also like that you can browse for 3 main criteria - relevance, popularity and how recent a photo is. don't forget as well as tags you can add explicitly, the picture title and a description are also used to rank an image. Just like in Google.