This is an interesting article on Google and how it grew in size:
Deja News (Google Groups) - This web-based Usenet archive started life in 1995. Between 1999 and 2000, Deja overexpanded into a comparison shopping portal. Losing money, Deja sold the shopping component to eBay in late 2000, and it became part of Half.com. In February 2001, the big G entered the game and snatched up the Usenet archives, reintroducing them as Google Groups and extending them back to 1981 with the help of private collections. Today, Google Groups features Deja's Usenet, mailing lists, and Yahoo! Groups-esque features with a Gmail-like interface.
Outride - Outride, Inc. was an information retrieval spin-off from Xerox Palo Alto Research Center (PARC). Google acquired certain technology assets in September 2001 and quickly integrated them into its search engine. Outride.net currently forwards to Google.
Kaltix - This 3-person personalized search startup company was quickly picked up by Google in September 2003. Kaltix formed the foundation of Google Personalized Search. Kaltix.com currently forwards to Google.
Blogger - Blogger was the flagship product of Pyra Labs. For a long time, Blogger was free of fees and ads, but it wasn't making money. After the original capital for Pyra dried up, a number of employees resigned, including the co-founder. In an effort to become profitable, Pyra introduced the ad-powered Blogspot hosting and the pay Blogger Pro service. It wasn't quite enough, and Pyra needed more resources, so Google stepped in during 2003. Blogger was redesigned by professional web designers in May 2004, and is now one of the most-used blogging tools.
Picasa - Picasa, a $30 photo organizer program, was first released in October 2001. In May 2004, Picasa announced integration with the Google-owned Blogger, and in July 2004, Google bought the company. Soon, Picasa was free, and it featured Google trademarks like an "I'm Feeling Lucky" button. The software routinely wins awards from leading PC publications.
Keyhole - Keyhole is a digital mapping company founded in 2001. Presumably to cut out the middleman for the not-yet-released Google Maps, Google bought them in October 2004. Since then, there has been an immediate price reduction for the Keyhole software (from $69.95 to $29.95), and integrated satellite photos in Google Maps.
Zipdash - Google acquired this traffic/mapping company in 2004 and put it to work in Google Maps. Although the acquisition was not publicized, Zipdash is mentioned in Google's 2004 annual report.
Where2 - This Australian mapping company was also mentioned in the 2004 annual report, but not much is known about it. It also had something to do with Google Maps.
Urchin - In March 2005, Google acquired Urchin, a web analytics and statistics company. Though we haven't yet seen what they're up to with it, it will probably be used with AdWords/AdSense, with statistics about clickthroughs and such.
Dodgeball - Google acquired this two-person cell phone social networking company in May 2005. The company was looking for investors, and Google apparently fit the bill. So far, nothing has happened with this company, but it will probably have something to do with Google Mobile.