We have used this heated chicken waterer from Tractor Supply for several years, and it works well:
http://www.tractorsupply.com/tsc/product/farm-innovators-heated-poultry-fount-3-gal
It can still form an ice skin on the coldest days, but it's never frozen solid and our girls have always been able to access liquid water--although I do check it each day to be sure. On the very coldest days, I will boil water in the tea kettle and pour it in the bottom well of the waterer to thaw any ice and give the girls at least short term access to warm water.
http://www.tractorsupply.com/tsc/product/farm-innovators-heated-poultry-fount-3-gal
It can still form an ice skin on the coldest days, but it's never frozen solid and our girls have always been able to access liquid water--although I do check it each day to be sure. On the very coldest days, I will boil water in the tea kettle and pour it in the bottom well of the waterer to thaw any ice and give the girls at least short term access to warm water.