This website looks pretty new, but if you want to warn others about a bastard ex of yours, or one who cheated on your friend, dontdatehimgirl.com appears to be the place to do it. Obviously, I'm only posting this because I don't see myself on there... yet.
This guy is Tom. We've been together for almost 8 years. I found his profile, using this photo that I took of him on our vacation 6 weeks ago, on a swingers website. He's also known to visit other assorted, less-than-desirable, dating websites. He markets himself as a warm and good person. BEWARE!{thx iBonka}