Eugene Zaikonnikov added a digital sensor from a Canon Rebel camera to his Leica M4 rangefinder. Detailed information, including source code, schematics and few sample photos can be found on GitHub. Additional details on the project are available on Rangefinderforum.
There is already a Kickstarter project for the development a 3D-module to convert a Leica M3 camera to digital by using the sensor of a Sony a7 inside.