Portal 2

Action, Adventure, Multi-player, Steam Achievements, Steam Trading Cards, Steam Cloud, Stats, Single-player, Co-op, Shared/Split Screen, Steam Workshop, Includes level editor, Full controller support, Captions available, Commentary available, Online Co-op, Shared/Split Screen Co-op, Remote Play on Phone, Remote Play on Tablet, Remote Play on TV, Remote Play Together, Steam Workshop, Family Sharing, Tradable

Link to the game in Steam

The "Perpetual Testing Initiative" has been expanded to allow you to design co-op puzzles for you and your friends!