Dokuwiki

Dokuwiki is an easy-to-use wiki software that requires only PHP.  It doesn’t save entries in a database, which means that it is:

a) Simple to set up

b) Easy to migrate to another solution later

It’s fast and easy to get started, and will allow your office to share ideas and documentation about processes, systems, and more very efficiently.

http://www.dokuwiki.org/dokuwiki