Many people, myself included, use martingale collars as a training tool with our dogs. For those who don't know what I'm talking about, this is a martingale collar:
These collars are great for training purposes when your dog is on a leash. Unlike "choke chains", there is a limit to how much the martingale collars can tighten so your dog can't "choke out" like with a choke chain. Also, when you give a correction, the collar tightens and then releases immediately, so there is no sustained tension around the dog's neck (and for those of us who have reactive dogs, we know how important it is to avoid tension at all costs!