OpenX Technologies, the open-source ad server that rebranded from OpenAds last year, has closed a $10 million third round, bringing its total investment to more than $30.8 million. DAG Ventures, a new investor, led the round. It was joined by previous backers Accel Partners, Index Ventures, Mangrove Capital, First Round Capital and News Corp (NYSE: NWS) digital media head Jon Miller, who also serves as chairman of OpenX’s board.

Aside from its rebranding, OpenX has made a number of changes since raising its $15.5 million second round in December 2007. In April 2008, it chose Yahoo’s former global ad marketplaces SVP Tim Cadogan as CEO. It also moved its offices from London, where OpenX was founded, to Pasadena, Calif. More recently, the company launched its OpenX Market, an exchange for “all classes” of buyers and sellers. Since last month, OpenX claims the market already has a monthly run rate of more two billion impressions.

Cadogan told me that OpenX plans to use the proceeds from the sale to supplement some cash left over from the second round as it looks to increase investment in engineers and sales personnel. “No one has built an exchange market for mid-size businesses,” Cadogan told me. “It’s a under-served part of the display business. And there is a general under-appreciation of display right now. But most marketing areas need the brand building that display offers, not search. I think companies are slowly starting to come around to that realization.”

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