DokuWiki is a simple to use and highly versatile open-source wiki software that doesn't require a database.
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Pro Free and open source
Licensed under GPL2.
Pro Easy to set up and mantain
To install you need a webserver running PHP 5.2 or later where you unpack the downloaded archive and navigate to install.php in your browser, fill out the necessary information required for the one-page installer and you are done.
Pro Plain-text file storage
Dokuwiki does not require a database, it stores everything in plain-text.
Pro Version control
Dokuwik offers unlimited page revisions.
Pro Good selection of plugins
DokuWiki offers over a thousand plugins to extend its functionality.
Pro Access control
DokuWiki has built-in ACL support.
Pro Runs on any PHP server
It requires a webserver running PHP 5.2 or later of any kind.
Pro Search functionality
DokuWiki allows searching through pages.
Pro A dedicated page for recent changes
A dedicated page to quickly note what has changed recently can be set up.
Pro Good selection of themes
DokuWiki offers over a hundred templates to change the visual appearance of the site.
Con Lots of plugins to manage
DokuWiki is highly modular. Even thing like WYSIWYG editors and categories have to be added separately as plugins. This can quickly lead to managing lots of plugins.