Less than a month after opening its display doors to third-party ad servers, Google has beefed up its list of approved providers.

Google has added Atlas and Tumri to its list of certified third parties. Atlas and Tumri join DoubleClick (a Google-owned company), Eyeblaster and Mediaplex as vendors allowed to put their ad tags on Google's network.

The program is also open to a select few rich media companies and research firms.

According to officials at Google, third-party participation will benefit advertisers and agencies by allowing them to manage campaigns more efficiently.

While Google has maintained that its decision to open the door to third-party ad servers was made only after the company was able to develop a rigid certification process, the move also comes at a time when Google is battling to establish dominance in the display arena.

Last month, Google began experimenting with serving display ads next to its search results, a practice made possible by the completion of its DoubleClick acquisition.

But gaining the upper hand against Microsoft and Yahoo in display could be a tough task for Google, which is facing growing advertiser discontent over the issue of piggybacking. While piggybacking -- the practice of competitors leveraging a brand's trademarked name or slogan by bidding on those terms -- isn't anything new, more advertisers are beginning to express anger over Google's inability to curb the trend.