Kayuda
"Kayuda is a web-based visual wiki, a mindmapping tool and a non-linear writing tool that allows you to track ideas and the relationship between them. Its free and any number of people can colloborate simultaneously in real time. You can have any number of "wikis" (called workspaces), and you can choose how private to make them".

PeanutButter Wiki

wiki.is

Wiki Matrix
Compare wiki software.

Zoho Wiki