Critters Workshop

For science fiction, horror, and fantasy, this is the best place I've found to give and receive critiques on fiction. The systems is set up to encourage use and participation by using a token system. You do so many critiques, and you earn the right to have your own work critiqued.

Other nice things about the website is the general information, their articles on how to give good critiques, as well as the discussion forums. The people there tend to be informative and imaginative. The only real downside is that sometimes one can get a little lost in using the interface for the critiques. However, once one has used it a few times it gets easier to find your way around.