Concrete5 : Site Building Toolkit (A CMS)

Concrete5 is a very powerful & open source content management application which has a different approach in how it works.
It enables users to manage websites while browsing it by simply switching to the edit mode (an overlayed screen). This makes anyone who can use a text editing application administer their websites.
The application uses PHP / MySQL & built with OOP.

With a very easy to use interface, new pages can added, updated or deleted.
A simple yet effective user permission/role feature helps creating admins, editors & other types of users.
There is a built-in versioning system which simply saves the old versions of a page & who created/updated it.
The application supports templates / plugins & there is a community getting more active everyday to get support.
To see the beauty of the application, check this demo.
P.S. Concrete5 is choosen the project of the month in October 2008 by Sourceforge.