Typo3
Typo3 is an open source content management system published under the GNU General Public License. It offers the fundamental functions of a CMS, like for example providing templates, user administration, life cycle management and modular extensibility.
Typo3 is a very famous CMS with several possible extensions. It is adopted world wide for about 60 000 websites in round 80 countries. The references show some big companies using Typo3, like BASF, ThyssenKrupp Automotive, Europcar, General Electric, Volkswagen, and many more.
MediaWiki
A wiki allows to its users to appear as consumers as well as producers in the world wide web. This means that the visitors of a website are not only able to read the site's content, but also edit it online directly in their Browser.
The content's treatment can be done using a simple markup language. The background for this is a reduced content management system. The most famous example is probably the online encyclopaedia Wikipedia, that uses the open source wiki software MediaWiki. MediaWiki is developed in PHP and licenced under the GPL. The contend is saved in a relational MySQL data base.
Some detailed instructions for creating and editing of articles can be found for example at the wikipedia cheatsheet.
WordPress
WordPress is a free software for Creating and Support of Website content. It's qualified for Creating Blogs, because every post can be assigned to one ore more categories. The corresponding navigation elements are also created automatically. In addition the software allows to assign tags to the created articles. Visitors can subscribe the articles via news-feeds and post comments to them.
WordPress is based on the scripting language PHP and licenced under the GPL. For saving the contents a MySQL database is needed. The software offers the possibility to manage user roles and authorisations, a media gallery with an integrated uploader, a central link management and a full text search. Per including of external plugins WordPress can be extended to an adequate content management system.
Magento
Magento is an open source online shop system. The E-commerce software is developed in the programming language PHP. Comparing to other shop systems like osCommerce or XT Commerce, Magento is a really new software and because of that fulfills current needs for dynamic online shops.