First cross-platform multiplayer puzzle videogame

Because bending the laws of physics is better with a friend, Valve designed Portal 2’s co-op mode to allow PS3, PC and Mac gamers to play together using the studio’s online gaming service Steam as a bridge between the systems. This was the first time a puzzle game allowed such robust cross-platform integration, and also the first time Steam was made available to PS3 owners. Sadly, Xbox 360 players missed the party and could only play with other gamers using the same console.