I've read a lot about the Tagg GPS collar, which works with a smart phone app. I wanted to let people know about the RoamEO Pet Monitor System. It's a GPS collar that works with a handheld receiver, and it's really affordable. Tagg has some great features, but there are monthly subscription fees. The RoamEO unit is $179 and that's it.
It's really great for times when your dog is off leash in a big area - hiking, vacations in rural areas, etc. You can visit the company's website http://www.amazon.com/Pet-Tronix-Roa.../dp/B004QWZ8UA
I think it's really great! It's so fun to see dogs run around without a leash, and nice to know that you'll be able to find them. Makes me want to go hiking!