What we've done to ensure drinkable water year-round is install an 80-gallon trough outside each of our run-in sheds, which are wired for electricity so we can plug in floating water heaters. We purchase these heaters, called Stock Tank Heaters, from our local farm-supply store, which sells them for about $35 apiece.
To discourage horses from playing with the cords and taking the heaters out of the troughs, we slide a 2-foot length of PVC tubing over each cord and snug the tube up to the heating element; the heater and the PVC-protected cord float on the water surface. We loop the remaining cord up the side of the tank and around a post next to the water spigot, then plug it into the electric box. Horses can't get their teeth around the PVC tubing or the looped cord to tug on the cord or yank the heater out. (The heaters have a safety feature, too: If removed from the water, they automatically shut off.) This arrangement has worked very well, keeping water available for all our horses throughout the winter.
I don't empty and clean troughs as often in the winter as I do in warmer weather, because the water freezes right where it gets dumped, making the ground dangerous for horses coming to the trough to drink. But every day I tidy each trough with a small fishnet (the kind you'd use for a home aquarium) that hangs by the electric box, dragging it around the trough to catch debris and keep the water relatively clean.
I continue my daily fishnet tidying as the weather warms, but I also empty and clean and refill the troughs at least weekly. (I wouldn't want to drink warm water that's getting grimy, and I don't imagine my horses would, either.)
For full details on Hope's system for letting her horses live outside year-round, see "Living Out 24/7" in the July 2004 issue of Practical Horseman magazine.