http://twiki.org (hosted at sourceforge.net) http://www.c2.com/cgi/wiki?TwikiClone
A very full featured Wiki aimed at corporate intranet usage, with full VersionHistory, authentication (although this is optional, you can have a TWikiGuest user by default), file attachments, etc. Now includes a plugin API, and is experiencing something of a Cambrian explosion of plugins for things like calendars, simple spreadsheets, action tracking, drawing, etc.
Seems to have a huge number of third party developers, though the "core" team is relatively small in number.