The Crew have the Hudson St Hooligans who basically have a corner of the ground to congregate, get drunk, and sing in. It really is fun, in a considerably-safer-than-any-football-ground-ever kind of way. Frankie Hejduk, as the captain, used to go over to that end at the end of the match, somebody in the crowd would hand him a pint, and he'd down it. It was ace.
I really hope they can keep that stuff going, because it's that that will keep/get people interested. There doesn't seem to be quite the same tradition of just singing all game as a group in other US sports.
http://www.youtube.com/watch?v=K_X8anlNYQk