Google has now started accepting third-party ads on the Google content network. Google will initially accept third-party ads in English only, but will expand to other languages soon. This will empower advertisers to work with approved third parties to serve and track display ads, including rich media ads, across the Google content network through AdWords, giving them more options, flexibility and control over their campaigns.
Making the Google content network more accessible to large brand advertisers also benefits AdSense publishers and end users. Third-party ad serving will introduce a greater variety of advertising into the Google content network, increase the inventory of quality display ads competing to show on AdSense publisher sites, and offer more engaging ads for end users. In the long run, we believe the increased inventory and ad competition will result in increased revenue for many AdSense publishers.[Google Blog]
How to control third-party ads shown on your site? The same ad management features for Google ads also apply to third-party ads. You can use the Ad Review Center to filter specific placement-targeted ads from appearing on your site. To block contextually-targeted ads from a given advertiser, you can use the Competitive Ad Filter. (The Ad Review Center currently only shows placement-targeted ads, not contextually-targeted ads.)