The New York Times has an article on Ask.com's Search Market share.  The most interesting thing about the article is not the content - I like the illustration of Search Engine Rank - more like a thermometer.

This representation is much better to look at than what I have seen in the past from ComScore.  About 74% of all search traffic is either from Google or Yahoo and if you add MSN - it's up to 85%.    But, since AOL Search Engine is powered by Google - the real number is 91% of all search traffic is covered by the 3 main search engines.