MediaWiki is the software used for Wikipedia (http://www.wikipedia.org) and the
other Wikimedia Foundation websites. Compared to other wikis, it has an
excellent range of features and support for high-traffic websites using
-multiple servers (Wikipedia peaks in the 1000+ requests per second range
+multiple servers (Wikipedia peaks in the 2500+ requests per second range
as of June 2005).
While quite usable on smaller sites, you may find you have to "roll your own"