6. Wikipedia
I would venture to guess when one of us needs to do a quick read on a topic we are not familiar with or are simply curious on some aspect of history, we pull up Wikipedia and do a quick search or browse.
Typically though, that quick browse can turn into hours as one article leads to another, and another. Naturally, there have been internet games created based on this phenomena. The wiki platform has been used as, or as a basis, of numerous projects, organizations, and political causes. The recent press over Wikileaks is a demonstration of the amazing potential this platform has, and will continue to have.
Controversy aside over the reliability of the articles in wikipedia, it simply is a ton of fun to use, and the potential for one website to have all the knowledge and history of the world contained within, make for an exciting navigation experience.
Fun Fact - "A match cut is any cut that emphasizes spatio-temporal continuity." - Wikipedia entry