I remember seeing a good one at a GDQ. It was an surgery game on the Wii. It was really cool to see the CO-OP version of the speedrun!

Grand Theft Auto 5 Online Grand Theft Auto 4 Multiplayer Grand Theft Auto San Andreas rampages CO-OP ( I LOVE GTA )

or Minecraft, Terraria, all Call Of Duty campaign CO-OP and more...

Portal has some coop runs as well.

Battleblock Theater

Contra seems really interesting to me

My attempt at doing 2 Player 2 PCs in Sonic Adventure 2: Battle with a_moustache.

What do I mean by this? A while ago he hosted an online marathon, and needed to use my PC to broadcast from as my hardware is quite powerful, so I gave him permission to remotely control my PC for the weekend to broadcast from.

During the planning stages of the marathon we were dicking around and discovered that if SA2B was launched, we both had control on our respective keyboards whilst he was controlling my system, albeit with about a second of input latency for him. We decided to try playing the game with me doing the directional inputs and him doing the action button inputs.

Didn't go so well. Was really fun, though.

Portal 2