7. Google Buys DoubleClick

Sale Price: $3.1 billion
Year: 2008

Google can afford to do nearly everything it does because of one business model: advertising. The company's AdWords and AdSense products bring in the vast majority of its revenues. In 2008, the Googleplex ponied up $3.1 billion to further its dominance of the online ad realm with the acquisition of DoubleClick—an ad-serving firm with a powerful client list. The price was reportedly driven upward by a bidding war with Microsoft.