There are a number of 3rd parties that help manage and optimize advertising on your website:

 - [Pubmatic.com][1]
 - [YieldBuild.com][2]
 - [RubiconProject.com][3]
 - [Admeld.com][4]

The benefit of using these services is that 1) you only need to implement one set of tags on your website, and 2) they dynamically optimize your ad placements to select the ad network that will pay you the most revenue.

  [1]: http://www.pubmatic.com
  [2]: http://www.yieldbuild.com
  [3]: http://www.rubiconproject.com
  [4]: http://www.admeld.com