Project M is one of the most impressive mods I've ever seen and this trailer for the upcoming 3.0 release on December 9, 2013 serves as a terrific introduction. The talent and dedication of these people -- my goodness! It's simply remarkable.
While the PMBR hasn't come out with the full list of additions, expect new characters like Mewtwo and Samus, the crazy-sounding Turbo mode, alternate costumes including Dr. Mario, and new/revised stages. That's to say nothing of the countless tweaks to gameplay.
If this is all sounding new to you and you're more of a Super Smash Bros. Melee fan than Brawl, you've got some reading to do.