James Randi, a great force of skepticism and reason.
James Randi has spent much of his life facing down and unmasking con artists and woo (dowsing, psychics, etc.). He has worked on his own, as part of his organization JREF, and with the likes of Johnny Carson.
You can look at some of his past work, and current endeavors at The James Randi Educational Foundation site.
Here you can watch video of one of his outings to show up the infamous Uri Geller, both appearing on the show, the View.
If you visit You Tube, their are plenty of clips of Randi exposing the cons.
James Randi Debunk Pt 1 - Looking back at this now, you hear Geller talking exactly like the people promoting the Secrets books and DVD's, on Oprah. Wanting = Getting. As if you needed another reason to dislike Uri Geller.
James Randi Debunk Pt 2
James Randi exposes Uri Geller and Peter Popoff - This is the above 2 segments with some additions.
James Randi exposes James Hydrick - This one is great, a real dismantling.
James Randi and Doris Collins "Cold Reader"
James Randi on Astrology
With a medical focus:
Jamies Randi Debunking Psychic Surgery - The blood is fake, but I thought I'd warn you.
James Randi Investigates Faith Healers - Has the last one on the end.
James Randi examines Faith Healing