DokuWiki is a standards compliant (http://www.webstandards.org/learn/faq/), simple to use WikiEngine mainly aimed at creating documentation of any kind. It is targeted at developer teams, workgroups and small companies. It has a simple but powerful syntax which makes sure the datafiles remain readable outside the wiki and eases the creation of structured texts. All data is stored in plain text files.
http://www.splitbrain.org/projects/dokuwiki
http://www.dokuwiki.org
DokuWiki is written in PhpLanguage, and is offered under the GnuGeneralPublicLicense version 2.
Some of its features: