The video linked to here has to be seen to be believed (and I dare anyone to watch it without laughing). An Australian octopus (Amphioctopus marginatus, to be specific) was seen gathering coconut half-shells discarded by humans and pulling them together to make a full sphere it can hide in.
This article is incorrect in saying this is the first known tool use by an invertebrate: wasps have been seen using a pebble to tamp down dirt. But it's amazing anyway. As one discoverer put it, what's so surprising is how the octopus seems to plan ahead: it will pick up a shell to use later, so "when it's transporting it, it's not getting any protection from it. It's that collecting it to use it later that is unusual."