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 mod is simply amazing. It changes what most people didn't like, and while some scenes might be a little choppy or odd, it does a great job.