Google and Wikipedia
Nick Carr speculates that, though Wikipedia is probably not the best source of all information, Google frequently puts Wikipedia on the top of its search results. Check out his rough research here.

One potential answer is that, in the early days of the web, Wikipedia was one of the most reliable sources of information. This was the cause of the high volume of Wikipedia traffic as users selected their pages over others in Google search results. I believe that the Google algorithm has evolved beyond Page Rank to include user click information resulting in further improvement in Wikipedia position in search results. It is a real challenge for others (even with better content) to get above Wikipedia. Let is see how Whorunsgov.com or the new Encyclopedia Britannica fare on Google in the future.

If Wikipedia were a "for-profit" company, it would be big and powerful and become a threat to Google itself.