Yahoo! has acquired the remaining stake in online advertising auction site Right Media, paying $680 million in cash and stock for the company.

The deal gives Yahoo, which bought 20 percent of Right Media in October, access to more than 20,000 buyers and sellers of online advertising. But the deal also helps Yahoo – a company many media watchers believe is facing troubled times ahead – an added tool in its battle with rival Google, which recently bought DoubleClick.

Yahoo CEO Terry Semel called Right Media a "very vibrant marketplace," in a conference call announcing the deal.

Forrester Research analyst Charlene Li wrote in her blog that Yahoo's acquisition, while appearing defensive, is actually a strong offensive move for the company.

"Yahoo is putting a stake in the ground that the future for online display advertising lays in efficient, easy-to-use marketplaces, and it wants to be the trusted intermediary for that future," Li wrote. "The acquisition makes it difficult for Google/DoubleClick to start its own ad exchange, which DoubleClick announced earlier this month. Right Media has been running its ad exchange for over two years, giving it the management and technical experience to run a successful exchange. But more importantly, I believe the acquisition puts pressure on Google's AdSense network to be more transparent."