Wikipedia.org

The people's encyclopedia, with millions of articles written in hundreds of languages. It's free, and anyone can edit. Its pages dominate Google search results, and the site is in the top 10 in terms of traffic. A vigilant group of volunteers helps maintain quality control. And now there's Wikia, where you can create a wiki of your own and get help managing it. Other offshoots include the Wiktionary, Wikiquote and Wikispecies, a "directory of life."