I don't have cats, but special lady friend does.
Cut down, I've found good use for them as short containers. These are being used to hold metal cutoffs and scraps that are too short for other storage.
Otherwise, basically anything you'd use a bucket for.
Otherwise, throw them on craigslists free section, and let some random person snag them for their use.
Otherwise, throw them away. Embrace the consumerist society we live in.