Linux.com has a review of DocuWiki. I've used/seen DocuWiki in the past, but for some reason or another I've never mentioned it here at CMS Report. DocuWiki runs on a flat file system (no database required) and I've known people to choose it over others due to its easy to use ACL (access control list).
Created as a simple solution for managing documentation, DokuWiki has evolved into a powerful and flexible wiki suitable for most tasks involving collaborative editing. DokuWiki doesn't use a database back end (all pages are stored as plain text files), which makes it easy to install and maintain. Its access control list feature offers a user-friendly and flexible mechanism for restricting access to certain pages and namespaces. You can also extend DokuWiki's default functionality using plugins, and there are hundreds of plugins to choose from.
If MediaWiki isn't your choice for a Wiki-only application, DocuWiki would be a worthy alternative.