A game I keep going back to. Quick to pick up, fun for all ages.
The critters you raise are called Norns: genetically engineered to be cute. They share a spaceship with the curious Ettins (engineered to be helpers) and Grendles (sort of a failed experiment, violent). You can get all sorts of things off websites like the Creatures Wiki to add new rooms, new breeds, and new objects (food, plants, toys, force fields...). You can export them to a file and send them out over the net to friends, share them with other ships/worlds, or save them in case a disease or genetic mutation wipes out your community.
Try Docking Station first: it's a free demo that connects to Creatures 3 and lets you trade creatures seamlessly over the internet. In fact, I think Docking Station is a must-have even if you buy the game. You can also buy Exodus: a bundle of Docking Station, several previously sold separately breeds, and C3. There is still an active community milling around, so pop into a search engine and look around at what we have to offer.