Today, Google has announced their Google Content Network is opened for third party ad servers. Now Advertisers and agencies can use the same ad serving system they use for the online campaigns for Google Content Network. Listed below are some of certified third parties ad servers;

1. Double Click
2. Mediaplex
3. Eyeblaster
4. Point Roll
5. Unicast etc...

There are more players coming in, read more at Official Google Blog: Opening our content network to third parties