And now, for something completely different:
Orange Mud endurance pack. Has a 2 liter bladder. Between that and two 26-oz bottles on the frame I can get about 60-70 miles on gravel depending on heat.
I've actually compared it directly to the Camelbak Chase that my wife bought and they're pretty similar but I prefer the Orange Mud; the design is just a little bit more well thought out in my opinion.
It's a small company so it doesn't get a lot of attention but they've been good to me. Plus I've chatted with the owner at various gravel events and he seems like a good dude.