I’ve seen Flickr mentioned a bunch of times, but I just played with it now: I’m really impressed. They do photo hosting plus a bunch of other stuff. If you sign up, they walk you through how to use the service (demonstrating features along the way) in a really well-engineered way. It’s worth a look.
(Warning: it uses flash, which probably is the best way to distribute web apps these days but also makes my skin crawl.)