They just passed the 100,000 entry mark. Essentially, everybody can use it, and everybody can edit every page. The project is based on wiki, a web tool that allows users to edit pages on the fly. I read about it the first time when it received the 2001 Jolt Award. This is a great way to put it to use!
… and there I thought the days of finding cool sites was over. I ran across the astonishing project wikipedia.org. It is a multilingual open content encyclopedia.