CSICOP On-Line -- home of the Skeptical Inquirer.

HillHouse Investigations-- Paranormal Detective Agency -- tougue-in-cheek Ghostbusters.

Encyclopedia of the Paranormal and Skepticism -- many definitions still to be added, but a good start.

The Skeptic Tank -- excellent articles on a variety of subjects. (Thanks to WitchesPromise for sending me this one.)