What's a good CMS for an intranet site?
Try OpenAtrium, its free.
If you're planning an intranet project using a CMS, then you'll need to clarify a couple of requirements before choosing the right one. I have a blog post with some simple choices for choosing a cms for an intranet, more specifically on collaboration and community features. But other more basic requirements are:
- Is there a technology stack that the organization prefers/uses? Does it need to be on-premise or cloud based? This will filter down the candidates
- Is the Intranet for just posting read-only notices and information, or are community features (groups, lists, news feeds, etc)
- Does the Intranet require SSO so that organization members can seamlessly interact with content based on their identity?
- What sort of budget is available for the Intranet? All CMS installs have a cost, even the ones without any subscription cost.
- Is document and file management an important requirement?
- Are customizations needed for any specific Intranet functionality or connectivity to other systems?
Wordpress will do a simple intranet well, but will start to become more work if you start getting complex requirements around authentications, groups and social functionality. If on the LAMP stack and looking for more complex requirements, look at Drupal or Joomla. On the Windows/.NET side there have been suggestions in this answer already - the choices span from full commercial answers such as Sharepoint to those available open source and commercially licensed like Dnn.
Nowadays everything is called a CMS - tools to maintain websites, advanced portals, wiki's, and so on. The requirements for a "CMS" are drastically different for intranets and public websites, however.
Intranets usually have a high level of interaction, lot's of user generated content, different content types, and so on. More users need to be able to login to the system (basically everyone, not just the content editors) with different levels of authorization and different roles in general. Collaboration in general is much more important than with an average "public" CMS based website.
Furthermore you will usually want different types of plugins. Google analytics and SEO are much less important, you'd be more interested in some active user plugin, recent publications, integration with other internal tools (i.e. project management) and possibly exposing other datasources (databases, telephone directories, filesystems with internal documents), and so on.
In my personal experience, Plone is a good choice. It provides most of the above out of the box or through existing extensions and it has excellent integration possibilities with external systems. Cyn.in also provides a somewhat completer plone based solution.
If Plone's too much for you, you could consider some wiki-like system, such as TWiki or MediaWiki