Google has filed its quarterly 10Q statement. Among the things in the filing is detail about Google's acquisitions in 2010. There were 40 costing Google a total of $1.6 billion and change.
There were three substantial acquisitions: "During the nine months ended September 30, 2010, we also completed 37 other acquisitions for a total cash consideration of approximately $626 million."
The big three were AdMob ($655 million stock, $26 million cash), Slide ($179 million cash) and On2 Technologies ($95 million stock, $28 million cash). The AdMob shares were either vested or on an accelerated vesting schedule because AdMob founder Omar Hamoui has left the building, only about five or so months after the acquisition formally closed.
Reference: Sec - Google's 10Q