One of the best ways that I've found to try and improve my photography has been to participate in photo critique sites. I really enjoy seeing on a daily basis the creativity that flows from photographers around the world. It inspires me to get out and shoot more. It also gets me thinking about what elements are common to the pictures that I like and what elements I would like to incorporate into my pictures. It's interesting to see the varied reaction to different shots, particularly the ones of mine that I upload. I've learned very very much from these sites and even though I'm more often a lurker, just soaking up the wisdom of others, it's a neat community to be involved in. The sites aren't without their flaws. Anything with a rating system inspires competition and hurt feelings. Nasty comments are often flung at people who critique without portfolios to back up their comments. People get kicked off. People voluntarily remove themselves. People come back with a better attitude after some time off. So, here's a quick rundown of the ones that I like.
Photosig - this is a very expansive site. Very well organized and judiciously moderated. The site with the most fighting but probably the most love there too. And, I once won a very nice Photosig mug.
Photocritique.net - The first site like this that I frequented. Not as nice to navigate but if you participate there is much feedback to be had. Lots of learning from there.
Usefilm.com - I don't check this site as often but when I'm there I'm often surprised by the quality of the images presented. Note to self, visit this site more often.
There of course are many many more, but these are the three that I go to the most. Then there is always Flickr, which is more of a community but you can get critiques if you seek them out.