Wiki with visual (wysiwyg) editing. Doesn't require users to learn another wiki syntax. Works on Mozilla and IE6. PHP + MYSQL implementation. Non-free license.
Also:
by DannyAyers (?) for standalone use (with heavy use of RDF) and integration into IdeaGraph http://ideagraph.net
See http://66.70.191.189/cgi-bin/mt-search.cgi?IncludeBlogs=1&search=stiki