Baking soda, 1 cup in 5 gallon bucket of hot water, labels float off with little work... Except earlier mentioned crazy insane glued labels. But MOST LABELS just float off or easily wipe away in baking soda after like 15 minutes.
Ammonia is the cheapest way. I used to use a couple cups in a five gallon bucket of hot water, and the labels would just fall right off.
Nowadays I use various oxygen cleaners, and they work just as well - but they are more expensive. I use these cleaners just because they work so well on cleaning gunk from the inside of bottles, also.
As far as labels go - ammonia is the cheapest way to do it.