Flickr has recently gotten even better as they continue to enhance the interface. It's getting to be more and more like an application. I used to think that the idea of web-based applications was sort of silly, probably because I use a lot of processor-hungry apps like Adobe Suite. But developers like Flicker and Google continue to prove me wrong with online versions of basic productivity software. And I must say that when traveling, it's very nice to have everything available through a browser.
But my new favorite is mostly for fun. Retrievr allows you to draw a sketch, then finds similar photos on Flickr. Very cool.