WordPress is a content management platform which is designed to make managing a website easy. Originally designed as a blogging platform, it has morphed over the years into a fully-featured content management solution suitable for many varied tasks and sites on the web. A healthy percentage of all websites on the Internet use WordPress in some form or another.
At TruthMedia, we use WordPress as our primary site platform. We extend it through many custom built plugins, some of which we have released back to the public.
WordPress website: http://wordpress.org/
You can also sign up for a free WordPress blog if desired at: http://wordpress.com/