Using the free Windows Sharepoint Services and SQL Server Express 2005 to build a company intranet, including document sharing, wikis, blogs etc.

http://thepaisano.wordpress.com/2008/02/01/how-to-build-your-own-social-network-in-the-enterprise-for-free/

Some templates for it here...
http://technet.microsoft.com/en-us/windowsserver/sharepoint/bb407286.aspx

A video of how to set up a wiki on WSS here...
http://www.youtube.com/watch?v=u2UKctgIjXg