It's a mod for Half-Life that comes installed with the game. I just gave it a shot, and it seems to be a blatant rip-off of Discs of Tron, only you play it in first-person. You can't jump yourself, but there are arrows you use to propel yourself to different circular platforms, and you throw discs in order to knock your opponent(s) off the platforms. You can also pick up powerups like triple-shots, decapitating discs (which kill instantly, instead of just knocking them back), speed shots, and freeze shots (you can also combine powerups. Fast decapitating triple-shots are scary
) It's a lot of fun, and my roommate and I have been playing it a lot lately.