it's a rather old concept, but i'm sure it could work in HL,and hasn't seem to have been done before in HL yet..
At the beginning, one player is designated the llama. this player has 1 hit point, and cannot use weapons, but they have double runspeed and jump height. the llama must survive for as long as possible, and earns one point for every second he is alive.
The player to frag the llama recieves 10 points, but he becomes the llama himself, so it's a choice between fragging other players, which earns one point per frag, or trying their luck at being the llama. Player with the highest score wins.