There are tons of photo sharing sites online, but the best of them has to be Flickr. I have been using the site for a couple of years now and am planning to take a professional membership, as in buy a membership, since the free accounts have several limitations. I have been taking photographs on a regular basis now, most of the time with uploading pics onto Flickr in mind, which is making me become more aware of my surroundings and take better pictures. It's certainly been quite an interesting journey.
Here's one I took last year.