I'm surprised this didn't already have a page.
Project M is a mod for Brawl that used to be to make the game more like Melee, the game's physics and gameplay feel exactly like Melee. (Is that a good thing? Who's to say?)
The game does something that is probably the best thing about the mod which adds new characters with new slots and they work perfectly.
The mod rebuilds most of the stages to make them more simple and fit for competitive play, but it still leaves in some illegals like Rainbow Cruise.
And then 12/1 it ended. The end.
I'm not kidding, a good chunk of the whole Smash fanbase considers Project M it's own separate video-game from the other Smash games. Is it really THAT good? Not really.
Also the current thought on its closing with the download link gone is that the dev team wants to get rid of its history with Project M so that Nintendo won't sue them when they make their own game, or something like that. And even though it's been semi-confirmed, people are still saying that that's insane and that they just closed the website and removed all download links just because.