Here is a very nice overview about the companies bought by Google in the last years and some prospects out there:

"Audioscrobbler/Last.fm - So far, Google hasn't made any inroads into the music industry. However, these sites together form an interesting, Google-ish service that uses algorithms reminiscent of PageRank to calculate the top artists and similar info.

TiVo - TiVo is a little too big and a little too well-known to be bought by Google. Also, Google's experience with hardware is limited to Google Search Appliances and similar. But, TiVo would work well with Google Video. TiVo seems to fit better with Apple Computer's media plans than it does with Google's geek mentality, though.

Icosystem - This swarm intelligence company might be useful for radical new spidering algorithms or some new form of PageRank. It's only peripherally Google-ish, though."

Nice analysis and the article will definitely give you a good idea about some interesting start-ups you probably have never heard of.