I have a "hands-free leash" for my dog - I just got a regular 6-foot leash (she's short, 4-foot might work for tall dogs), and put a clasp on the end with the loop, so that I can loop it around my waist and clip it there. You can also just loop the leash around and step through it, so it's looped around your waist (hope that makes sense). It's wonderful for me, I love having my hands free. But I'd be a little nervous to put it on a big, tall dog - they're stronger and get better leverage, making it more possible to hurt your back or pull you down if they do pull. But, if your dog is good at loose leash walking and doesn't pull (or if you have him on a head halter or something), I'd definately recommend the leash.
I know that there are a few hands free leashes out there, the one I'm thinking of is on petsmart.com I think, it's just a belt you put around your waist, and then you attach a leash to the belt somehow. I didn't like it because then I have an ugly belt around my waist all the time, but maybe it would work for you if you don't mind that. A double-ended leash would be perfect, but please let me know if you find one, I've only seen a few and they're custom-made.