Yes, I've saved those, too. Another item I save is the little white plastic box the Cesar's dog food comes in. Since it's the only thing my little dog will eat, I've had stacks of them & then I have to just toss them out. They don't have lids though.
I also love those sturdy plastic black trays that precooked frozen meat from Costco comes in. Kills me to have to finally part with them when they stack up.