contextweb2.jpgThe online advertising market is consolidating at a terrific pace, leaving fewer significant independent companies around.

The ranks of ad exchanges, for example, have slimmed considerably. Google acquired DoubleClick, Yahoo gobbled up Right Media, Microsoft has snagged AdECN, and marketing conglomerate WPP’s bought 24/7.

ContextWeb’s ADSDAQ is one of the last major independent online ad exchanges in the market,.

Ad exchanges are different than ad networks. While ad networks collect inventory from a number of publishers and sell it to advertisers, ad exchanges are marketplaces where publishers put the inventory up for sale and advertisers (including ad networks, themselves) bid to buy it from there.

ContextWeb says ADSDAQ has a special difference. It provides publishers the ability to set the price for their inventory. ContextWeb says that by putting price-control power into the publishers’ hands, ADSDAQ gives publishers a strong incentive to sell their premium inventory on its exchange. The other ad exchanges typically deal in remnant inventory, helping publishers unload the ad space that hasn’t already been bought through other means (though Right Media’s Mike Nolet takes issue with the distinction between “premium” and “remnant” inventory).

ADSDAQ is also different because it has “open doors.” On July 9th, the company initiated a private beta of a self-service system for publishers of all stripes. Once this service goes public, any publisher will be able to put its ad space on the exchange. The company argues that this makes it the only “long-tail” ad exchange in the game.

This is all fine if advertisers are willing to pay the publisher’s asking price. A lot of the time, though, it is the advertiser that drives the transaction.

In September of last year, the New York City-based Conextweb raised $15.5 million in its third round, bringing their total raised to more $27.5 million.