Spore
Co-op for Spore, made by WoodyWizard. Play the stages together in one shared world, each with your own creature. You need Spore and Galactic Adventures from GOG or Steam, current builds only; other copies start without co-op. It replaces steam_api.dll with its own loader (the original is kept), so some antivirus programs warn. Known limits from the author: the Steam online overlay can hide the mod menu, EA app and disc copies are not supported, and it has not been tested over the internet on Windows.