4. AdMob

Price: $750 million

Announced: November 2009

FTC Approval: May 2010

    The purchase of AdMob is consistent with the strategy already exhibited by this list: Google aggressively ramping up its advertising inventory. Along with Invite Media and AdMeld, this plunged Google into mobile advertising. Mobile apps are an increasingly important venue for ads -- just ask anyone who plays Hanging With Friends.

    Google announced in May that in its first full year with the company, AdMob grew by over 3.5 times, getting over 2.7 billion daily ad requests. The company plans to use the AdMob network in further integration with DoubleClick's ad tools to ultimately operate on a single platform across all devices.

3. YouTube

Price: $1.65 billion

Announced: October 2006

FTC Approval: November 2006

    The video giant wed the search giant in late 2006 to mostly positive feedback. Analysts noted the move, though expensive, gave Google "a leader in two of the big three, email, search, and video," as Goldman Sachs wrote in a report from the time of the announcement. The biggest concern experts had at the time of the sale? How Google would address copyright concerns with the user-driven video site. Google has managed to deal with such copyright issues successfully, while keeping YouTube relevant and looking forward. Corollary purchases such as On2 will allow Google to continue to innovate -- and make more money -- from the site.