Wikipedia passed 2 million English language articles today. This free, user-generated encyclopedia has been around since 2001.
I've got to hand it to Wikipedia, despite it's politics and the hole Jimbo Wales thing...it's a cool site and often helpful. Sometimes, it's surprisingly thorough and I like how it lets you know when it's not. (e.g. "this article is a stub", when it lacks references, citations, etc)
All in all, Wikipedia is a solid representation of what Web 2.0 is about. Good and Bad. High Five's Wikipedia!