Russian Rocket-Propelled Grenade launcher, 'RPG'
Born as a light, cheap and mainly anti-tank weapon, it practically was the terror of any motorized vehicle and even some very light buildings too.
It had no electronic guidance, but it automatically exploded on the target proximity thanks to an infrared thermal sensor. So, in order to destroy a moving target, the user needed no perfect accuracy.
It had little effectiveness against the infantry, because it's warheads produced no fragments, but it could destroy any helicopter and most of armoured vehicles.
So much was a widespread weapon that any US vehicle was considered at risk at the simple sight of a single Vietcong within RPG range.
Just like the AK, the RPG is still a current weapon, and is still used nowadays as then, using the very same tactics and giving the same results.