Umbraco is a free open source Content Management System built on the ASP.NET platform.
The easiest way to get started is to run build/build.bat which will build both the backoffice (also known as "Belle") and the Umbraco core. You can then easily start debugging from Visual Studio, or if you need to debug Belle you can run grunt dev in src\Umbraco.Web.UI.Client.
If you're interested in making changes to Belle make sure to read the Belle ReadMe file. Note that you can always download a nightly build so you don't have to build the code yourself.
More than 350,000 sites trust Umbraco
For the first time on the Microsoft platform, there is a free user and developer friendly CMS that makes it quick and easy to create websites - or a breeze to build complex web applications. Umbraco has award-winning integration capabilities and supports ASP.NET MVC or Web Forms, including User and Custom Controls, out of the box. It's a developer's dream and your users will love it too.
Used by more than 350,000 active websites including http://daviscup.com, http://heinz.com, http://peugeot.com, http://www.hersheys.com/ and The Official ASP.NET and IIS.NET website from Microsoft (http://asp.net / http://iis.net), you can be sure that the technology is proven, stable and scales.
To view more examples, please visit http://umbraco.com/why-umbraco/#caseStudies
The downloadable Umbraco releases live at http://our.umbraco.org/download.
We have a forum running on http://our.umbraco.org. The discussions group on Google Groups is for discussions on developing the core, and not on Umbraco-implementations or extensions in general. For those topics, please use http://our.umbraco.org.
If you want to contribute back to Umbraco you should check out our guide to contributing.
Another way you can contribute to Umbraco is by providing issue reports. For information on how to submit an issue report refer to our online guide for reporting issues.
To view existing issues, please visit http://issues.umbraco.org.