http://www.game-advertising-online.com/An advertising agency specifically targeting game sites!
So if you have a browser-based game with over 10,000 impressions per day, you can sign up for this CPM ad program! Starting rates of about $0.05 per 1000 impressions.
Looks alright, but I guess with other ad agencies you can earn more money... The program lists some big browser games as their publishers. Check out the Site List page.