I have the one made by REI; I think it's called the Piggy Back. It has been a very nice pack. It has a comfortable suspension, a decent-sized pack for carrying stuff (other than the kid), a rear view mirror that stuffs in a pocket (so you can look back and see the kid). It is also hydration ready. I've carried about 30 pounds in it without a problem.
My son is 18 months old now, and is tall for an 18 month old, and he still fits in it. He likes going out for a hike in it. He fell asleep a couple times, so it must be pretty comfortable.