DotNetNuke is a content management system (CMS) for websites, easy to use and highly extensible, with lots of add on modules.
One of my customers made me use Joomla. After suffering with that for three years, I was not motivated to look into another CMS, especially since I had written my own Asp.net framework, which of course I loved (and want to port to DotNetNuke).
After peering at DotNetNuke for a year, I finally gave it a try and immediately saw how powerful and easy to use it was. Have been moving my customers' and my websites to it since.