MEHEM is a reinterpretation of the endings of Mass Effect 3, which allows Commander Shepard to survive the final mission, among many other changes. It features custom videos and slides, re-engineered gameflow, original as well as re-arranged voice acting, new music, etc.
This is the installer for MEHEM v0.4, the current version of the Mass Effect (3) Happy Ending Mod. Simply download and extract the archive and run mehem04.exe. Readme files and an uninstaller, which will set Mass Effect 3 back to it's original state are included. In case of problems or for further information, please check the MEHEM thread on BSN.