The first was Deja in February 12, 2001 and the last was TalkBin in April 26, 2011.
Most of the acquisitions were made to use as a Google product or to integrated in one or several.
Take the example of Google Docs:
- April 17, 2007 Tonic Systems - Presentation program
- June 19, 2007 Zenter - Presentation program
- December 4, 2009 AppJet (EtherPad) - Collaborative real-time editor
- March 5, 2010 DocVerse - Microsoft Office files sharing site
Full list of acquisitions here.