Here in Maine, 2x6 walls are generally insulated with R-19 fiberglass insulation. There's this other stuff I've just run into though called Roxul; not sure the R value, but it's better than fiberglass, more expensive, flame retardant, and I've watched water run right off it. I think the old term for it was rock wool, and it's been around since the 1920's.
Maine is about to make new requirements as far as I can tell, but I can't remember off the top of my head what the new R values have to be. As I understand it though, there is already a fiberglass insulation out there that will achieve the new R.
There's also spray foam, but I've got no experience with that. We had one person in our area who was doing it for people, but he's either in big trouble or been chased out of town; I can't remember which.