While researching Snake Oil I came across two very interesting sites that I figured deserved links in their own right:

The James Randi Educational Foundation is run by James "the Amazing" Randi, a guy who's been debunking psychics and other charlatans for years.

Miss Cleo Unmasked (scroll down a bit to get to the actual article) is an interesting first-person account of someone who actually worked for our favorite valley-girl-turned-Jamaican-psychic.

Both well worth a look. Enjoy!