Main Post: Free and Useful Web Applications

Web Wikis:

MediaWiki
MediaWiki is a free software wiki package originally written for Wikipedia. It is now used by several other projects of the non-profit Wikimedia Foundation and by many other wikis.
Free: Yes
Platform: Linux
Review Rating:
Homepage: http://www.mediawiki.org/

PmWiki
PmWiki is a system for collaborative creation and maintenance of websites. Pages created with pmWiki look and act like normal web pages, except they have a link that makes it easy to modify existing pages and add new pages into the website.
Free: Yes
Platform: Linux
Review Rating:
Homepage: http://www.pmwiki.org/

TWiki
TWiki is a flexible, powerful, and easy to use enterprise wiki, enterprise collaboration platform and knowledge management system. It is a Structured Wiki, typically used to run a project development space, a document management system, a knowledge base, or any other groupware tool, on an intranet or on the internet. Web content can be created collaboratively by using just a browser. Users without programming skills can create web applications. Developers can extend the functionality of TWiki with Plugins.
Free: Yes
Platform: Linux
Review Rating:
Homepage: http://www.twiki.org/