Open Atrium is an open source platform designed specifically to make great teams communicate better. It's an intranet in a box with: a blog, a wiki, a calendar, a to do list, a shoutbox, and a dashboard to manage it all. It’s also completely customizable, and available in more than thirty languages.
Features include: a classic blog for each group that has commenting, file attachments on both the post and comments, and granular notifications; a calendar that lets you quickly add events, suck in iCal and other calendar feeds, and support single or multi-day events; a simple online handbook that lets you collaborate on documents, store and compare revisions, attach files, and once you're done print out the final copy; a full ticketing system that lets you assign to do's and create unlimited projects within each of your groups; and more. You can also easily build your own features or use features created by other members of the Open Atrium community.