Starting to think about what the new Intranet is going to be. Initially the concept of a Wiki seems appropriate with broadly the following areas:
1. Corporate. The "official version" type information, ie published policies, procedures, news etc which is published from "the company"
2. myTeam. While you may be a member of many teams this sort of content is ment to be user generated/updateable.
3. myStuff. Information published for you by you.
The one problem with Wiki is it isn't really how lots of people use the web, for that you need more of the social networking stuff and here is a list of open source social networking platforms that could play into the intranet space. Key strength is that its the information you / your collegues believe is important and may include information from outside the corporation.
Have a look at this article from 2007. http://mashable.com/2007/07/25/open-source-social-platforms/
and this comparison on wikipedia. http://en.wikipedia.org/wiki/Comparison_of_social_networking_software