A good one is cgsociety.org
Edit : http://forums.cgsociety.org/ to be specific. Since you're looking for a forum that offers you great critiques and will help you improve, this is a good site, not only to promote yourself, but to also learn more. There's a whole subsection for different techniques, be it animation, lighting, matte painting, etc. It's an entirely artist oriented forum and you'll be sure to find an abundance of artists of different skill levels to help and critique you. In addition, there's always a newbie thread for just beginning out, and a "personal anatomy and sketchbook thread" much like an ongoing openended gallery for critiques and keeping all your roughs in one place.