There are a lot of options:
You can use a CMS like Joomla or Drupal
You can use a Wiki tool like MediaWiki or Dekiwiki
You can use SharePoint server ( basic version comes free with Windows Server 2003 - check me on this)
You can see an exhaustive list here:
For CMS:
http://en.wikipedia.org/wiki/List_of_content_management_systems
For Wikis:
http://en.wikipedia.org/wiki/List_of_wiki_software
Personal Recommendation and Experience
I used Joomla and DekiWiki for at 2 companies, and tried a bunch of other ones: Drupal, TikiWiki, MediaWiki, Cayahoga, Xoop, etc.
I like DekiWiki the most - for the ease of use, extensibility, features. It is open source and they have a free and GPLed edition, but if you need help - you can always fall back on their paid support. ( They were also number one project on SourceForge at some point). They have an Enterprise version, but to be honest after using Community version for almost a year in a environment with about 150 users - I really don't see the need to upgrade.
Also one more thing: I used these tools in big corp. environments - and while for personal use I like RedMine ( redmine.org) , a lot of users - find DekiWiki the most easy to understand and use.
Also DekiWiki search is based on Lucene.