Fish is an aquarium simulator. It allows you to add fish to an underwater environment and customize them.
Fish show complex behavior. You can feed them, see their evolution as they grow, use different cameras to track them, and save different aquariums for later usage. Fish can be shared with others over the local network using Rendezvous.
It keeps the kids busy enough & out of trouble. There is room for improvement with graphics & more customizable options, but for a free application it's more than just good! So it still deserves top marks from me!