I went to a Google techtalk at MIT yesterday, and they showed a Google project that I'd never seen before: Google Trends. It shows you a graph of the query volume for particular phrases. You can enter multiple phrases to compare them, and it also labels the graph with related news reports. Here's some interesting charts:
Python versus Ruby:

Wikipedia versus Java:

Slashdot versus Digg:

Blog versus news:

Posted on September 14, 2006 11:09 AM
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Posted by: Sqcovldlti at October 22, 2007 03:19 AMYes it's a great fun tool for measuring the zeitgeist.
Try doing a Google Trends comparison on Google, Yahoo and Microsoft !