Wikipedia is both an encyclopedia and a wiki community. You can edit articles on Wikipedia right now! Learn how to contribute with the tutorial (or just play around in our sandbox). For more information, post comments at the Village Pump, read the Help directory and our policies, contact other Wikipedians, and keep track of what’s going on. –Wikipedia: Community Portal (Wikipedia)
I’ve created or added to a handful of Wikipedia articles over the years (usability, interactive fiction, Elia Kazan, Rossum’s Universal Robots), and I’ve also (briefly) had students work on articles for brief exercises.
One day I hit the the “random article” button and found myself reading a creepy but very informative article about snipers… then a few months later, when the Beltway Sniper was in the news, I felt very informed about the whole matter.
It looks like Wikipedia has recently added community and magazine-like features, in an apparent effort to compete with pay sites like the one for Encyclopedia Britannica.