Wikipedia is probably the most famous wiki.  This page in Wikipedia compares many different available wiki engines. Comparisons are made across general and important features, target audience, and the installation if you want to host on your own server.

http://en.wikipedia.org/wiki/Comparison_of_wiki_software

Welker’s Wikinomics, created for a high school economics course, has won two awards for best in education. Check out what types of information is included and how the information is accessed through different navigational controls (a top banner tab system, a left-hand menu, and various embedded links). A ‘Rules of Conduct’ is even included for the student as well as a blog that is accessed from the ‘Discussions’ tab. The Course Information page includes a rubric for student grading.

Home page: http://welkerswikinomics.wetpaint.com/

Course information page: http://welkerswikinomics.wetpaint.com/page/Course+Information

Wikia hosts many wikis sorted by their type of content, that is by entertainment, gaming, or lifestyle. You can connect with people interested in the latest hit television show, with fellow World of Warcraft gamers, or with cooking enthusiasts, to name a few. The home page provides a nice summary video of why to use a wiki. Be sure to visit a few wikis to see how content can be presented in a variety of layouts, colors, and navigation schemes. You can also create your own wiki through this web site.

http://www.wikia.com/Special:LandingPage