Open source intranet solution - Open Atrium
Open Atrium is an open source intranet package that uses Drupal as it's underlying framework.
As Open Atrium is a Drupal installation profile, it runs using PHP and MySQL, which means other than hardware cost it is free to get started and run. It features a few familar modules to any Drupal user such as Views and Organic Groups but also introduces some lesser known modules such as Features and Spaces.
It is currently in beta and is currently at beta 3.2 which can be downloaded the from Open Atrium website.
You can install it like you would any Drupal installation, but during installation you are given the choice to install Drupal standalone or Drupal with Open Atrium's modules and configuration preinstalled.
Features include a blog for updates, a calendar for events, documents which uses the books module to produce a hierachial display of all the documents you want to store, a shoutbox and a case tracker which is ideal for development teams. Each feature can be turned on or off within each organic group which means you can easily divide internal teams in to different groups to keep the intranet a bit tidier than it would if it was all in one place.