Pushover: like Summer said, it doesn't automatically pair players, it just lets you play with whoever else is currently also online wanting to play.
Ahh, sort of how the old Yahoo Games ladders worked? Those were easily gamed though, it was impossible to move up the ladder because all the good players refused to play anyone that could make their rating go down. It was less about skill and more about who you played, and it was impossible to crack into the highly rated player clique.
How do you plan to combat this? Penalize players who refuse matchups? Some other method?