I have been working with the content management system (CMS) Joomla for more than two years and it has to be one of the most powerful free applications on the planet. Not only is it a world class CMS, but you can mold it for so many uses. For example, I use it to host this web site, I have used it to build a topic specific social network, and I am currently evaluating it as a learning management system . If you would like to take a look at how others are using Joomla, make a visit to JFoobar . They have a great run down of many sites that run Joomla.

One of the greatest assets Joomla possesses is the development community that supports the application. Not only does this community volunteer their time to code and support the core application, they have produced an enormous number of extensions that increase the functionality of Joomla beyond your needs and wants.

Here are few examples of the extensions that have been developed, and most for free: 

Social Networking
Newsletters
Surveys
Learning Management
eCommerce
Microblogging
Multimedia
Chat
Facebook Connect
Open Social

If you are charged with building web portals that involve managing content of any type, I recommend seriously evaluating Joomla.