Google has published special version of AdSense for the online games publishers. Online games are getting popularity day by day since those can be played from anywhere and no installation is required. Flash has make online games so popular. Multi-player online games are very popular.
With AdSense for Games, you can display video ads, image ads, or text ads within your online games to earn revenue. You'll be able to show these ads in placements you define, such as interstitial frames before a game, after a level change, or when a game is over. You'll see contextually targeted text and image ads based on content and demographic information. In addition, you'll be able to control the ads you see on your pages using filtering options.
The program is in beta stage now. So it's not still open for everyone.
This will give the game makers opportunity to earn money still offering play games for free.