photobucket has a photo-sharing widget that you can plug into your site, allowing your web site visitors to upload, access, and share images from their Photobucket account.

Granted, users need a Photobucket account to use it ... I wonder if the widget makes it easy & transparent to sign up?

I posted this because MyPunchBowl lets you share photos from flickr, but if you don't have a flickr account, you're out of luck.