Chivalry: Medieval Warfare
I decided to be a nerd and check the entire forums to see if we had it and we don't, the game is also based on multiplayer, and most likely would have a good potential when it comes to hacking it. It's basically an FPS, but without the guns, and instead melee weapons, like javelins, spears and so on. Overall, I reckon this game would have a great potential by means of hacking.
Cheers, The Fapping Dark Lord