ummmm....how about using them as planters outside this summer?? you could put some holes in the bottom for drainage and voila!
You could save a couple of the clean/in good shape ones and use them as tubs for scrubbing stuff off in...I'm always in need of a scrubbin bucket (I have landsnails and cleaning their tank isn't the easiest thing in the world).
That's what immediately comes to mind, but I'll keep on thinking.
I donated one to holding birdseed, because the cats ate through the bag.
I've also used them as trashcontainers for litter and litterbox waste. Traps the smell for weeks.
Or, if you want to be really nice, call up the local shelter and see if they could use them. We use them for food containers for foster parents. They're refillable and sturdy enough to tote around.
If you are talking about the plastic tubs, I used those to hold the dirty litter after scooping. With the plastic lid, it held in the smell until it was full. When I emptied the entire pan, I'd pour it into the plastic tub.
If you are talking about paper type containers, I just use those as garbage bags.
I have a bunch of plastic buckets from years ago when I'd buy their food in buckets. I use those to hold craft items, painted some and used those to hold kindling by the fireplace, store chemicals in the garage that I don't want critters to get into, and use one as a compost bucket under the sink. When you live in the country, you NEVER throw away a plastic bucket!
I use some of mine to store potting soil or other various types of gardening items (perlite/vermiculite/fertilizer/peat moss ) my husband uses in workshop to hold shims. I use also to hold different types of birdseed. I make stuff out of concrete so I use to store my sharp sand/portland cement. Basically anything that come in a large bag I will dump into these containers.