There are a number of open source content management systems and tools being used to build web sites today. Among the top that I see mentioned the most are WordPress, DotNetNuke, Drupal, and Joomla. I'm also seeing Umbraco mentioned more in conversations as well because it supports .NET.
While not everyone has likely heard of all of these, many people have. It was just announced that Joomla has reached a milestone of having been downloaded more than 25 million times.
Of course, part of what makes a successful open source and public CMS is the ecosystem of add-ins and add-ons. For Joomla, there were 2,714extensions registered on Joomla's extension directory, with more than 8,150 extensions being reported as available.
What is great about products like Joomla, is that it gives organizations a quick start into building a web site. I am an organizer of a local Give Camp that spends a week-end building solutions for non-profits. At the IndyGiveCamp the developers are often start with tools like Joomla and Drupal in order to build a solution for charities in a week-end. It is a testament to the tool that such a feat is possible in such a short time.
It won't surprise me to see the numbers for Joomla downloads continue to grow!
Shameless plug: We promote a number of Microsoft products on our sites. One product we promote for Microsoft is WebMatrix. Microsoft WebMatrix is a tool that lets you build websites. It includes the ability to use Joomla and then to customize it through the WebMatrix tool. You can download WebMatrix here.