I don't know what these things are called, but I've seen plenty of parents walking around the parks with a harness type thing on young children. It's basically like a leash that you attach to the child's upper body, and you can always hang onto the leash and know where your child is.

I used to think these things were terrible, but now that I have a two year old, I'd rather have a leash looking thing on her than have her lost in MAJOR Disney crowds - too stressful for all of us!!

What's your opinion on those leash things?
What exactly are they?
Where can you get them?