

There aren't too many things that are great and come free in this world. But there are a few open-source applications, specific to the content-management environment, that could be a great fit to help you build an intranet in your business or organization. And of course, what help makes them a great fit is that they are free, and since they are open source code, they are customizable from the code base up.
I've listed the ones I personally know of here, and if you know of any others that have successfully been applied to an intranet specific purpose, please contact me and I would be happy to add them to this list.
Drupal is another content management system, or CMS. This type of application will enable you to create an intranet homepage quickly and painlessly. The best part is: hundreds of plug-ins and add-ons will give you as much functionality as you can handle.
If your intranet needs it, Drupal probably has it already. If you need a login system, look no further. If you want to display news, blogs, or other feeds - it’s there. If you need any type of chat, email, instant messaging, form to mail, or other communication integrations - again, Drupal has it.
What sets Drupal apart from other CMS designs is that it is just so easy to use and maintain. Making a clean looking design, adding modules, content, and setting up accounts is no problem at all. If you plan on having interactive CMS content, be sure to give Drupal a good look too.
Joomla is probably one of the easiest CMS's to install - you just create a MySQL database and follow the web-browser based instructions after that. Joomla was designed to be easy set up even if you're not an advanced user. If you are using off-site web hosting, many offer a single-click install, which would enable you to get your new site up and running in just a few minutes.
If you need specialized functionality, Joomla is highly extensible and there are thousands of extensions (most for free under the GPL license) are available in the Joomla Extensions Directory. Arguable, Joomal is one of the most popular open source CMS's available today.
You could create a full featured intranet with this CMS, and I recommend you review the features available for implementation. I think Joomal is getting better and better all the time, and has a solid core of developer support.
Plone is an open source content management software that has been available since 2001. It has been supported for many years now and boasts almost 100 developers in the project, with tons of user add-ons available for enhancing your deployment of this application.
Plone is basically ready-to-run, you just perform the one-click install and start customizing, with the ability to customize the look & feel with your own templates. Plone is built on the free Zope application server. The benefit here is that Plone can operate with most relational database systems, both open source and commercial, and runs on a wide variety of platforms, including Linux, Windows, Mac OS X, Solaris and BSD.
Plone is easy to set up, very flexible, and provides you with a system for managing web content that is ideal for project groups, communities, web sites, extranets and intranets. You will find tons of extensions that will very capably create a full featured intranet for your use.
These three open source content management applications all have the potential to fulfil your requirement to create an intranet for your company or organization. Now, they key is to research the three and see which have the add-on capabilities and ease of use that are the best fit for you!