Unlike many of the other sites, I feature here, Critters has been up and running for quite some time and has a very good reputation. If you don’t write fantasy, horror, or sci-fi, then check out Critique, which houses critique sites for any art form – from poetry to photography.
Critters seems to be geared towards writers who are serious about their craft and who really want to improve. Andrew Burt, who runs the site, has put together a guide on how to give “diplomatic” critiques. It’s a great guide and is good for all kinds of critiquing, not just on Critters, but on all websites (and in real life, of course); and not just for writing, for all types of art.
So give it a shot!