I got the idea from the Far Cry 3 video where the player sneaks up behind someone and pulls the pin on their grenade and they go boom :D
It would work like this:
If a player has a grenade in their inventory, and if another player sneaks up and presses a certain button they will activate their grenade and after a few seconds they will be blown up. There could be an option that makes the player shove the other player so he has time to run or just knock him into a group of players.
/discuss