http://www.mediawiki.org/wiki/MediaWiki
This one is the server behind the wikipedia, thus its stability and scalability is guaranteed. However it does not support WYSWYG, which may scare some inexperienced users.
MoinMoin (Apache, Ubuntu, Fedora, etc)
http://moinmo.in/
Instiki
http://www.instiki.org/show/HomePage
Instiki (What Is Instiki) is a wiki clone so pretty and easy to set up, you’ll wonder if it’s really a wiki. In fact it’s not only a wiki. It supports file uploads, PDF export, RSS feeds, multiple users, multiple wiki sites and password protection. Some people even use Instiki as a CMS - Content Management System because of its ability to export static pages.
Tikiwiki
http://info.tikiwiki.org/tiki-index.php
TikiWiki (Tiki) is your Groupware/CMS (Content Management System) solution. Tiki has the features you need:
- Wikis (like Mediawiki)
- Forums (like phpBB)
- Blogs (like WordPress)
- Articles (like Digg)
- Image Gallery (like Flickr)
- Map Server (like Google Maps)
- Link Directory (like DMOZ)
- Multilingual (like Babel Fish)
- Bug tracker (like Bugzilla)
- Free (LGPL)
