At the Big Media & Enterprise meetup in London, the firm Helpful Technology presented on how they built an intranet to serve various departments within GOV.UK, the often praised website of the UK government.
Helpful Technology has a working demo of the intranet, and the code (including custom plugins) is on Github. Their team further describes the project in a 13 minute video and slides on the WordPress.com VIP blog.
Intranets are a big opportunity for WordPress (hello SharePoint!), and this is a good case study for study and reference, not to mention free code to explore.