Yes, it's the web based encyclopedia that is free for everyone to use, and contribute. Given the fact that anyone can change almost any article in it, it is truly amazing that the quality of the information in it well rivals commercial encyclopedias. Kudos to Wikipedia!
Here are the other interesting Wikis that I have encountered.
- Encyclopedia Dramatica - Another parody of Wikipedia, apparently the Wikipedia admins got so irritated with it that they "took it personally", and subsequently deleted the Encyclopedia Dramatica entry from their site!
- Uncyclopedia - "Uncyclopedia, the content-free encyclopedia that anyone can edit, is a satirical parody of Wikipedia, though Uncyclopedia claims the reverse." - Wikipedia
- Simple Wikipedia - This a simpler version of the Wikipedia designed for slower (mentally challenged) people.
- Memory Alpha - And finally here it comes, the Star Trek Wikipedia.