AppFuel has just launched an Ad network for Facebook Apps. They bill themselves as an 'intelligent ad network' and they specialize in dealing out targeted, relevant ads that reflect users' actual tastes and interests. It's meant to provide a win-win situation. Appfuel also works quickly; they'll develop a highly targeted ad in a matter of seconds. Advertisers and publishers are their primary clients; however they do have a self-serve dashboard which allows anyone to advertise their apps on Facebook. Publishers will get paid via Paypal. For more intensive campaigns as well as info on rates, you'll have to get in touch with the Appfuel team.