Drupal is industry leading content management system used to manage websites, admin panels and perform a lot of other web-admin functions.
WordPress is a web-standard CMS written in PHP and MySQL database as a server side plugin. Popular since its launch, with largest user base.
Umbraco is an open source ASP.NET-based content management system, with a beautiful, user-friendly interface, full support for your own .NET controls.
Another offering from the open-source world, DotNetNuke is a web application framework for creating, deploying and managing interactive websites.
An ASP.NET web content management platform developed by Telerik, and the only CMS to consider mobile development strategies by default.
Kentico is a comprehensive and proprietary web CMS aimed to create websites, stores, community forum sites and corporate intranets..