Cuphead
Cuphead co-op: both cups, the host's save, bosses, levels and the world map shared online (Steam invites, LAN fallback). Originally made by Germanized (with Sh0kr), maintained by Multiplayer Everything. Known issues: 3 or more players is experimental, shops, equip cards, pause, Dice Palace, Ms. Chalice and plane levels are not covered by the lab yet, and Steam online play still needs a two-account test.