Any new puppy or kitten we got spent nights and times we were away closed in our bathroom. That is sorta like a crate I guess... We never thought of it as "training" just making sure we knew where the little things were and protecting them from all our grown (and generally pretty nasty) cats. It was our way of gradual introduction to the house and the other pets.

We son't actually own a crate or anything. The only animals that are in cages are my rats (and even they get several hours to romp around the room every day).