Deadheads
Trekkies
Elayne Boozler fanatics
Recycling obsessives (we call them Trash Enthusiasts.) Don't throw me under the bus here; I am not saying that conservation is unimportant, just that, for the most part, Trash Enthusiasts have not come up with a palatable way of educating that. The term "shrill" comes to mind.
I was just joking about Elayne Boozler fanatics; is there one?