Return to Castle Wolfenstein 3D is a mod for registered Wolfenstein, made by Corrigan Jr and Geese. It was released on March 3, 2012.
The mod features 60 levels, and tries to blend the atmosphere of 2001's Return to Castle Wolfenstein with that of Wolfenstein 3D. Many new graphics and sounds have been added to compliment this.
Most levels are quite detailed (although most of them was borrowed from elsewhere), and the RTCW atmosphere is definitely felt. The mod was four years in the making.