Tear Gas

In real life, tear gas is generally considered to be a non-lethal weapon, but in the world of GTA, it's anything but. Granted, it does take prolonged exposure to the gas to kill, but in confined spaces, this can be arranged. However, seeing as the gas causes people to stop what their doing while they cover their face and cough, the most practical use for this weapon is to slow enemies down while you escape or switch to another weapon. In any case, this stuff opens up some strategy options when it otherwise seems you're heavily outgunned.