Keeper Collars (martingale and hidden prong collars)
Herm Sprenger Prong Collar
Julius K9 Super Grip Leashes
This collar operates using blunt medical grade stimulation (like a physical therapist TENS unit) rather than a sharp shock. Tone and vibration are also available on these collars. Ordering from this site directly ensures that you are getting the latest collar rather than a discontinued model. These collars can also be found on Amazon (just check to be sure it is the latest model). There is a chart midway down the page that compares models. Dogs are able to swim while wearing these collars.
The Leerburg company also sells the mini educator collars
There are 4 videos on this page showing the mini educator being used and explained by various people.
Dog Bark Collar
Tools for deterring loose dogs on your walks: