I definitely enjoyed Deschutes Black Butte Porter and Ninkasi Oatis stout when I was in the states, but I think I was just a fan-boy for any Oregonian beer. thenewgreen mentioned Moose Drool, and while it's not what would initially come to mind as far as dark beers go, it's a damn good ale. Very drinkable, yet still satisfying.
Yeah, it looks dark but it definitely doesn't drink like the typical dark ale and definitely not a stout, but Moose Drool is a pretty good beer by my estimation. -Could be more nostalgia than anything. I used to drink the Black Butte Porter quite a bit too. Great beer.
Perhaps a bit more than just nostalgia. I thought it was quite nice, and more than a few of my American friends have listed it among their favourite beers. I found it fairly similar to White Rabbit Dark Ale, for the Aussies playing along.