Project M is the premiere modification for Brawl that brings in Melee's fast-paced physics including L-Cancelling and Wave Dashing. It brings balance changes to every character to make each one tournament viable.
The game brings back veteran characters, Mewtwo, Roy, and Dr. Mario (as a costume, due to Mario and Dr. Mario's movesets being combined together). It also brings in a plethora of competitive stages including HD remakes of N64 stages. The game has a casual play aspect of it as well, which retains non-competitive stages and its own mode called "all-star mode" which allows the player to play a different character after every stock within a match.
Project M ceased development in 2015 due to being in a legal grey area, and has been banned from Twitch streams, large tournaments such as APEX and EVO (even though it was on the main stage in previous years), and mentioning "PM" on Miiverse (now discontinued) would result in being reprimanded for "illegal activity". Despite this, tournaments continue to host Project M but only under certain conditions. One of the Ex-PMDT (Project M Development Team) members leaked a build of Project M that was never officially released to the public. It contained new characters Knuckles, Lyn, Isaac and Sami. It also contained some new alternate costumes, artwork, and stages. Almost none of this content is allowed in tournaments. What is allowed is specific content which helps tournaments, and costumes and other cosmetic changes not present in the leaked builds. People want to see Knuckles in tournaments, however, or other leaked content but they are forbidden from doing so in fear that everyone would get in serious legal trouble if they were to be used (even though they were already leaked to the public). Also, balance patches are strictly forbidden which everyone generally agrees on in order to not change the meta game.