I looked into these as well...
Amazon.com: GARMIN 010-11049-00 Astro DC 30...
Thought it was interesting. It's kind of a GPS collar but I think it allows you to set certain boundaries for your dog and it doesn't shock them but it does beep and it shows you where they are on your phone or computer, etc.
Currently, I live on 3 acres unfenced. I have a retractable stake in the ground that goes out 20 feet and I then attached a 50 foot nylon leash to that so he gets 70 feet to roam. It works for us for right now but I'm still trying to find a better solution.