Wikipedia, of course.
But this article from MSNBC points out that Wikipedia isn't always the ultimate source for the ultimate truth. When Senator Ted Kennedy was hospitalized after suffering a seizure Tuesday, Wikipedia was the first to post the news...of his death. It took several editing tries for Wiki's avatars to correct Kennedy's entry and lose the section of his life story where he died.
The article's author poses an interesting claim here: even though Wikepedia serves as an encyclopediac reference, it's the first place people look when they want to know whether something's true. Why check CNN or MSNBC for late-breaking news when the information you want is available (though at the end of the entry) on Wikipedia? The piecing of information from multiple sources into single comprehensive entries appeals to us. However, incidences such as this serve to remind us that Wikipedia isn't infallible.