I'm willing to bet that nearly all of us have one of these things laying around. It's one of those desktop coffee cup warmers. I put it on an upside down plate and put a ceramic bowl on top of it. Temps have fallen into single digits here, and it keeps it from freezing. I've tested the temperature several times, and it keeps the water at about the average temperature of tap water. Needless to say, the hen's love it.
I keep it plugged in 24-7 with an extension cord. I change the water daily. I only have 5 hens right now, so it works for me.
Edited by Preservation Acres - 1/7/10 at 4:43pm