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WordPress shouldn't need any introduction - it powers over 30% of the top 10 million websites. In total, it's used by more than 60 million websites. It's managed this impressive feat with a focus on ease of use, and ease of expansion. Thousands of plugins and themes are available to customize the look and functionality of your site in any way imaginable.
Drupal has been around since 2001, and by recent estimations, powers somewhere around 2.3% of websites worldwide. Drupal can be seen as more complicated than both WordPress and Joomla, but that complexity comes with increased flexibility. With it's steep learning curve, Drupal is suited to large complicated websites where there will be dedicated developers.
TYPO3 is an Enterprise Open-source CMS, built with businesses in mind. It's first release was in 1998, and is used to power some 500,000 web sites. It's strengths are it's business focus, security & stability. It's weaknesses are it's smaller community, and not so extensive extension and template libraries.
Joomla is the second largest CMS by market share, but even so it trails WordPress by a significant margin. It's fairly easy to install and get set up, but beyond that, the admin interface can be daunting, to say the least. That being said, master the complex interface, and you'll be rewarded with a powerful, and very flexible CMS.
Initially released in 2003, and powering over 1 million websites, DNN (formerly DotNetNuke) is the premier CMS for Microsoft's .NET Framework. DNN is configurable and extensible with a fair number of plugins and themes available, it has also been built with security at the forefront.