OpenX has reported raising another $10 million in Series C funding, bringing the total investment in the open-source ad server to more than $30.8 million.

Many investors from previous rounds pitched in again, including Accel Partners, Index Venture, Mangrove Capital, First Round Capital, and digital media head of News Corp Jon Miller, who is also chairman of OpenX's board.

The round was led, however, by new investor DAG Ventures - a rarity in these troubled times, TechCrunch observes.

OpenX - rebranded from its previous name Openads - plans to use the funds to expand its products and services, and invest in engineers and sales personnel, reports PaidContent.

Last year the company launched the OpenX Market, an exchange for "all classes" of buyers and sellers. Web publishers on the market that are about to run an ad from an advertiser paying $1 for the ad, for example, can see if someone else will pay more for that space, via a real-time auction. If a higher bidder exists, OpenX automatically runs that ad instead - and takes a 15% cut.

Some 38,000 publishers use OpenX, which powers 150,000 websites. The market has a monthly run rate of more two billion impressions, says the company, citing last month's data.

And there is room for growth. The ad market for mid-size businesses is under-served, and though display ads are under-appreciated right now, companies will realize that they need the brand-building that display offers, said CEO Tim Cadogan, a former Yahoo executive.

"We have a very large number of publishers and a decent amount of inventory, so we use our very extensive ad-serving footprint to pool together inventory and make it easy for people to buy,” he told The New York Times.

OpenX's first round of financing ($5 million) closed in May 2007. It raised $15.5 million more by the end of the year.