If you've contemplated deploying Microsoft SharePoint (or Microsoft SharePoint along with Jive SharePoint Connector - for a more socially-inclined Intranet), you may not need to shell out what will inevitably become thousands of dollars. In fact, depending on what you'll be using it for, the best option for you could very well wind up costing you zero dollars and zero cents.
Open Atrium, while not as complex as Microsoft SharePoint (which can be both a blessing and a curse), has just about everything you need to get started with your company's very first Intranet solution. In fact, the authors tout the software as an "Intranet in a box."
Right out of the gate, you'll be able to collaborate with other employees, teams and departments using the six major features of Open Atrium: document sharing, calendars, to-do lists, wikis, blogs, a comprehensive dashboard and a pretty thorough shoutbox.
This open-source Intranet solution is runs off of a number of different (all free and also open-source) technologies including MySQL, PHP and Drupal.