Another upvote for Code Sponsor! Been using it for almost a month or so, and it's pretty awesome.
Styfle already describes it very well, so I'm just gonna add that although I'm seeing a very shaky impression/click rate, and my clicks are below what I expected to receive, the payout is quite big for an ad/sponsor network like this. Also, it's much better than ad networks - no detailed tracking and more (listed below by cavneb)!