Lately I love Flickr. I could browse it for hours - although I try to exercise some self-control.
Also, in connection with Flickr, I love this site: Big Huge Labs, which has all kinds of cool apps on it for doing quick and easy photo projects. Its designed to work best when hooked up to a flickr site, but if you don't have one, that's no problem either; you can upload any random photos you want as well.
Here are some of the things we've made on Big Huge Labs: