I've always know every Dunkin' Donuts to be open 24 x 7.
Actually, McDonalds has highly-rated coffee and is always a safe bet on the road. Sometimes 24 x 7.
Denny's has been good wherever I've tried it and 24 x 7.
If you're looking for an "environment" to sip coffee, however, NOTHING, absolutely NOTHING beats the Frontier.