I recently started posting pictures at TrekEarth, a travel oriented photo sharing site. I like it because it's more of a "serious" photo site, rather than a place to put up pictures of you doing something stupid over the weekend (ahem...Flickr...not that there's anything wrong with that site...it is where we put up a bunch of pics).
One of the features is the opportunity to receive critiques by other members. It's an opportunity to learn more about the art and science of photography. So far I've got about 10 of my best pictures on the site, and have been working on preparing others. You can only upload one per day, so it forces you to cull through the thousands of good pictures to really pick out the best ones.
Check the site out. http://www.trekearth.com/
The link to my Trek Earth page is in the links on the right...