Umbraco - simple and friendly ASP.NET CMS
Umbraco provides a full-featured web content management system that is easy to use, simple to customize, and robust enough to run the largest sites such as wired.co.uk and asp.net. It is most popular and widely used CMS. It has more than 85000 active installations.
It has loads of features required for a CMS. It does provides support for Polls, Forum, Podcast blog, Event management, Access control and permissions for pages, WYSIWYG editor, Displays twitter feed and post, Searchengine friendly, Gallery, Paypal support, Personalization of content for logged-in User and lot more.
This open source product can be bundled with guaranteed and professional support, bug fixing warranty and productivity enhancing add-ons.
Source code location: http://umbraco.codeplex.com/