The Rubicon Project, which essentially acts as a middleman between web sites and ad networks, announced Monday that it raised an additional $15 million in funding, bringing its total financing to $21 million. The recent round was led by Mayfield Fund along with returning investor Clearstone Venture Partners, plus IDG Ventures Asia, Stanford University, University of California Berkeley, and Matt Coffin, founder and former CEO of LowerMyBills.com.

“We’re creating the world’s largest ad sales team by leveraging ad networks,” said Frank Addante, CEO and founder.

The bulk of the additional capital will go toward international expansion as well as acquisitions and further infrastructure development, said Mr. Addante.

The Los Angles, California-based startup helps web sites manage and optimize multiple ad networks in order to generate higher ad revenue. The free self-service site simplifies the sometimes confusing online ad landscape for publishers, providing them with consolidated billing across ad networks and an online dashboard that lets them see earnings, reporting, and click rates.

“If a blogger writes about flowers and makes 300 dollars, and then writes about chocolate and makes 1,000 dollars, that blogger might want to write more about chocolate,” said Mr. Addante. We’re giving web sites “visibility about ad space that they never had before,” he said.

Still in beta, the site claims to have signed up more than 3,000 web sites since its October 2007 launch. Customers range from small blogs to Reunion.com and Beliefnet. According to the Rubicon Project, customers have seen revenue increases from 33 percent to 300 percent.

This is not Mr. Addante’s first foray into online advertising. Along with other members of the Rubicon Project team, he created the successful ad platform L90/adMonitor, which was later acquired by DoubleClick.

The success of Mr. Addante’s previous ventures, along with the growing advertising market, helped attract investors, said Raj Kapoor of the Mayfield Fund.

There are “few solutions helping publishers maximize inventory,” said Mr. Kapoor. According to Mr. Kapoor, 80 percent of ad inventory online is not sold directly by web sites. “What’s left goes to ad networks, but they may not be able to monetize it the best,” he said.

With Yahoo’s acquisition of Right Media and Google’s plan to acquire DoubleClick, the large ad networks appear to be aligning with big internet players, which may or may not be optimal for publishers and advertisers.  “Rubicon is a neutral party,” said Mr. Kapoor. “Google may be conflicted by wanting to keep a publisher on their network.”

The complexity involved in choosing the right ad network also poses a problem. For small companies, learning “how DoubleClick works is a bit bewildering,” said Jeffrey Lindsay, a senior analyst at Sanford C. Bernstein. And for companies who hope to reach an international audience, figuring out the best ad network option can become even more confusing.

Given this situation, “there is a role there for an intermediary…to identify advertising opportunities that would be difficult for a company to do on their own,” he said. “This is very far from a crazy idea.”