Bz Flag

BZFlag, short for Battlefield Zone Flag ,is a open-source freeware multiplayer first-person tank action game.
 * Tanks can jump. But they cannot change direction midair as they retain momentum before jumping.
 * Every tank have fixed forward gun and have One Hit Point Wonder. How much you can shoot at one time depend on server setting.
 * On default server setting, bullets ricochet off walls. This can lead to hilarious suicide.

Power up items called "Flag" are scattered around the map. Either beneficial one like Laser,Guided Missile,ability to change direction midair or nasty one like reversing your turning control, forcing you to change side temporary or even blacking out your screen. It helps that this game have a radar/minimap.

Bz Flag provides examples of:

 * Hit Scan: Laser flag.
 * Homing Projectile: Guided Missile flag. God help you if you're on different height than it's user.
 * One Hit Kill: Rampant.
 * The Wiki Rule: Right here.