Open-source startup OpenX Technologies Tuesday is expected to announce closing a Series C financing round worth $10 million, and led by DAG Ventures.

DAG, along with existing investors -- Accel Partners, Index Ventures, Mangrove Capital, First Round Capital and Jonathan Miller, the company's Board chairman -- are now responsible for helping OpenX to raise roughly $30 million.

OpenX is planning to use the additional funds to further expand its ad technology products and services, and to accelerate the growth of its recently launched OpenX Market.

"Even in the midst of a tough economic climate, there is huge opportunity for companies that creatively address large challenges such as those currently facing online advertising," said Tim Cadogan, CEO of OpenX.

OpenX bills OpenX Market as a simplified marketplace for all classes of online buyers and sellers. "If Yahoo's Right Media exchange is structured like a financial market, OpenX Market is more analogous to a search marketplace," Cadogan told Online Media Daily.

And Cadogan should know. Before joining OpenX one year ago, he spent five years at Yahoo -- ultimately serving as its SVP of global advertising marketplaces, including display, search and video.

Publishers are now being encouraged to route their ad impressions into the OpenX Market through tools that are already integrated into OpenX's established ad server. After publishers define a minimum "floor" price for their ad impressions, the market then runs a real-time auction for each impression.

Also setting OpenX apart from other ad markets is the fact that it does not set a minimum fee for publishers to participate.

If the winning bid from the auction is higher than the publisher-set minimum price, the higher-paying ad is served and the publisher makes more money. If the winning bid is less, meanwhile, the publisher's original ad runs.

Headquartered in Pasadena, Calif. with offices in London and Poland, OpenX's primary product is an independent ad server, which is used by over 35,000 publishers on more than 150,000 Web sites online.

The company estimates that roughly 300 billion ad impressions run through its OpenX software every month -- about a third of which come from U.S. Web users. Rivals include the DoubleClick-backed Google Ad Manager, AdMeld, Pubmatic, and the Rubicon Project.

OpenX -- then known as Openads -- raised $5 million in its Series A financing, which closed in May 2007. It later raised $15.5 million in its Series B financing, which closed in December 2007.