According to an article by the Register, 97% of illicit search queries in China, as determined by governmental censors, are filtered by Yahoo.  Sometimes the search engine even imposes a time-out penalty:

In fact, researchers found that searching for “Tibet independence” or “6-4″ (4 June is the anniversary of Tiananmen Square) won’t just give you shonky results but will get you barred from the site for an hour.

I must point out that censorship of the internet is simply fighting a losing battle.  If people are saying that your government sucks, the solution isn’t to prevent other people from hearing them, but to make your government un-sucky.