The online ad market is crowded and very entrenched with Google's AdSense and DoubleClick controlling a fair share of it. Then there's Microsoft and Yahoo! And even they are having trouble competing with Google. So you would think that there wouldn't be much space for a start-up to grow. However, open-source ad server OpenX did just that and has now secured $10 million in venture capital funding to help it with future growth.

Run by ex-Yahoo! Executive Tim Cadogan and former AOL CEO Jonathan Miller, the ad company has gained significant market share recently despite the global slowdown in online advertising in light of the struggling economy. It has seen a growth of 500% to a monthly run rate of 7.5 billion Internet page impressions in the December 2008 - April 2009 time frame with 10,000 new users of its latest product. In total it has more than 300 billion impressions a month with a network of about 150,000 sites using its services, which makes it comparable to DoubleClick in terms of volume.

OpenX has changed its name several times, most recently from Openads, and was developed by a community of open-source contributors over the course of nine years. It has already gotten two rounds of funding for a total of $20.5 million with the latest being in December 2007 when it attracted $15.5 million. Now it has raised capital mainly from a new investor, DAG Ventures, with current investors Accel Partners, Index Ventures, Mangrove Capital and First Round Capital also joining in.

CEO Tom Cadorgan declined to comment on the valuation of the company after the latest round of funding. He noted, however, that a large part of the investment from December was still left and available to the company and there wasn't a need to attract more money.