While I was building my wikis, I looked at a lot of examples to find what I liked-and what I didn't. What I found was that there are SOO-OO many educational wikis out there! This wiki link has MANY examples of educational wikis across all grade levels, disciplines, from all over the world.
Some of the wikis I found had so much information that it was hard to navigate the real purpose, and they seemed more like a class website than a place to collaborate effectively. Here is one example from a 2nd grade class. While this wiki is easy to navigate, the collaboration is hard to follow. Each topic on the sidebar takes you to a taxonomy where it seems the class might have collaborated on ideas, pictures and links, but it is hard to tell.
Another wiki I found was geared towards teen readers. This wiki has not been updated in a while, but it clearly was a collaborative place for teen readers to share their thoughts on many different genres of reading. It is not the most visually stimulating, but it hits on some of Vicki Davis' other elements of an effective wiki. It has a collaborative effort (teens write book talks), the book talks are organized into genre links, some readers provided hyperlinks to outside sources, and contributors were original in their reflections.
I think that wikis certainly have their place in the classroom. It can be a wonderful way for students and teachers to collaborate, but I don't think it should be confused with a class website. I think collaboration is the key, and a wiki should clearly show how students have worked together to create the learning space.