Huddle is quite promising and i have tested it out a bit. There are things I wish they did a bit differently but that may be just me. I think a site like this for indie game devs would be great. Easy to share new content and get new content while maintaining a timeline.
Well, there's Alfresco http://www.alfresco.com/. Can be set up for free, from what I gather. But the more features you want in a system like this, the deeper you have to dive into paying people to set it up - at least usually.
Also, http://www.huddle.com/ might be a lead. Never really tried either.
I'm actually considering solutions for a number of things as well, right now lots of islands of information that don't really integrate all that well.
MediaWiki, especially with the VisualEditor extension, is remarkably capable for sharing information too. I'm on a bit of a MediaWiki high right now, upgraded it when I needed to upgrade my own internal documentation wiki and the latest version with the wysiwyg editor added is pretty impressive. I was fairly blown away when I took another wiki I had and just copied and pasted the text - including fields that were preformatted (code and the like) and pasted it into VisualEditor... and it got it 99% right, headlines and all.
Add the right calendar extension (if they exist - I know there has been calendars that could tie into Google Calendar for instance) and it might serve some function for you. AD integration can be very complete, can even sync groups and such off AD.
You want to have a look at Mango Apps (http://www.mangospring.com) and Huddle (http://www.huddle.com). Confluence is also a good call, and Sharepoint can work well as an add on to many of these, for document repository and versioning, Project Server and the like.
We've just gone with Mango for our 'knowledge sharing platform' but it was a really close call between that and Huddle. The default CSS for their site makes it look quite a lot like a combo of G+ and Facebook, which drives employee adoption as it's something they're familiar with and doesn't feel so much like 'work'! Really good for the Generation Y lot that are coming into companies now, giving them a method of communication over and beyond the comparatively ancient and static method of email. Also, Gartner just added them to their Cool Companies 2012 list.