Like many (most?) people, the first wiki on my radar screen was Wikipedia, and until recently it was the only one. Wiki encyclopedias alone make for a fascinating study: There's the vast Wikipedia; and the new Britannica with its combination of authority articles and participant contributions; and the upcoming Google "Knol" which will permit anyone to post signed articles but none will be editable so you might have 500 entries on the same topic. And those are just the general encyclopedias. There are many more wiki encyclopedias devoted to particular topics, all with their own rules, almost all having a mission to deliver high-quality information free of charge from a community of experts to the public.
The 23 Things #16 page has some cool links to other wikis. A Subject Pathfinder example, where I was able to plug in someone's name and approximate age and find out his birthday! Great for stalkers. *hem* A book-lovers' wiki at the Princeton Library, where patrons post reviews: limited, but you can see the appeal for the community using it. The ALA conference wiki. Public library wikis, such as the one in Loudon County, Virginia, an old stomping ground--fun to see what's new there. And if you click on the link "Other library wiki examples," you get a terrific source that features a multitude of innovative ways to use this collaborative tool, not only in libraries. This seems like a tool that is being deeply mined for its potential. It's definitely here to stay, and worth knowing how to do. (Segue into Thing 17...)