Creatures 3 / Docking Station Stuff
I never did much with Creatures 2, but the third game in the series and its online-capable add-on recaptured my imagination by flinging our hapless norns and grendels into space. Not only did I make more objects for the game, but I programmed tools to help me make those objects. The scripting language for Creatures 3 is incredibly sophisticated, to the point where even the user interface lives in the game world itself. It remains one of the most popular entries in the series, especially for those pushing the envelope of norn behavior and genetics, world-building, and ongoing experiments and wolfling runs.