Watch a five minute introduction video

http://umbraco.org/documentation/videos/getting-started/what-is-umbraco

Umbraco - the simple, flexible and friendly ASP.NET CMS

For the first time on the Microsoft platform 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 got award-winning integration capabilities and supports your ASP.NET User and Custom Controls out of the box. It's a developers dream and your users will love it too. Used by more than 57.000 active websites including Heinz.com, Peugeot.com, NAIAS.com and Microsofts documentinteropinitiative.org website you can be sure that the technology is proven, stable and scales.
More info at http://umbraco.org

Umbraco 4 is out - Best. Web CMS. Ever :-)

We're proud that the final release of Umbraco 4 is out. This is recommended for production and ultra stable. Download from releases tab

Forums

We have a forum running on http://forum.umbraco.org. The discussions area on CodePlex will be for discussions on developing the core, and not on Umbraco-implementations or extensions in general. For those topics, please use http://forum.umbraco.org.

Contribute to Umbraco

You can submit patches to umbraco from the source code tab. Frequent patchers can be considered for the core team. If you want to join, start by solving issues and keeping an eye on core team chat announcements!