Right now the only game that is supported it RTCW.
There are two major sites that are doing it. I don't remember either of them. Wired ran a thing with three reviewish things, then MaxPC ran an article blurb on two of them.
More or less, you pick one of their servers, (nickel, quarter, dollar, etc.) then its a 1 for 1. You die, you lose a dollar. You kill, you win a dollar. The site takes something silly like 17% of what you win.
I assume you pay up before you start playing, ala "credit".
Pretty dumb IMO.