I'm a developer and other things, and I take a particular interest in implementing intranets. I refereed a book on building intranets, and authored two technical titles with Packt Publications, the first of which built the world's first open source employee intranet photo directory, and the second of which built a dashboard.
One recommended practice is not to build an intranet for the sake of building an intranet or using a sweet tool. Instead, come up with a business motive and objective that can be addressed by building an intranet, and build an intranet that is a tool for meeting that goal.
With that stated, there are some pretty sweet tools that I can fairly easily put on an intranet. These include:
Drupal is a multipurpose content management system.
Hayward's Free Intranet Employee Photo Directory treats a gap in open source coverage for a photo directory.
Koha Library Software is a library management system that is top-notch if you have a library.
MediaWiki is the same software as Wikipedia runs. An intranet wiki can be a very useful tool where information that is out of date doesn't stay out of date for terribly long.
Moodle is an open source learning management system, that unlike Blackboard is based on a pedagogy. If you have employee training, Moodle may be able to help you.
phpBB is an online forum and bulletin board solution. It can foster excellent internal discussions.
Request Tracker is an industrial strength request handler and ticket tracker.
SuiteCRM is a customer relationship management system.
TikiWiki is a feature-rich all-in-one system.
WordPress isn't just for blogs. It's a content management system, although internal blogs can be helpful.
I can put your selection(s) from these on a virtual machine that a sufficiently powerful computer can run from your network, for $50 per system included. I can also do additional custom work to customize an intranet built for your needs.