I use my voice. You don't have to be all that accurate on tone, so long as you do right on the cadence. I can do it well enough to get barred owls pissed off at each other-- that's when the real fun begins. Once you get the owls in on it, then they honk off their neighbors and so on. One time I did that up in Northern Ontario, and I swear I must have had every bird above the 45th parallel sounding off.
Generally, I only owl early in the year, when I'm trying to get turkeys to gobble during my scouting. By the time season starts, I have the birds pretty well patterned, and let the ambient sources do the work. If you're patient, the crows, owls, woodpeckers, etc. get the gobblers going plenty.