Google Inc. Friday announced a highly anticipated service that will make it a middleman for selling graphical ads over the Internet.

The technology, called the DoubleClick Ad Exchange, resembles a stock exchange for display ads. It allows companies that buy ads to bid for ad space across different Web sites in real time based on what publishers want to sell that second. Today display ads are often purchased ahead of time through negotiations with individual Web sites or networks of sites, a process which leaves publishers...