The WordPress answer works well if you want a "static" intranet. However a lot of businesses are opting for a Wiki-style site, where it is collaborative. The more that employees contribute and add to help others, ultimately helps the company.
An OpenSource (OS) (free) site to consider would be: www.openatrium.com: Open Atrium is an intranet in a box that has group spaces to allow different teams to have their own conversations. It comes with six features - a blog, a wiki, a calendar, a to do list, a shoutbox, and a dashboard to manage it all
Another option is: TWiki® is a flexible, powerful, and easy to use enterprise wiki, enterprise collaboration platform, and web application platform. 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, extranet or the Internet.
I hope this helps. I've only listed two. We use OpenAtrium and it works well for us.