Sling

Tens of millions years ago out pre-human ancestors figured out that throwing rocks at other beings hurts them and it allows you to stay away from their teeth, claws or tusks. It took quite a long time and some evolutionary changes to the brain to improve the concept, though.

Meet the sling - the most ancient ranged weapon known to mankind. It it's very basics a sling is just a rope with a cloth or leather pouch in the middle. You put a rock in that ouch, fold that rope in half and give it a good swing before releasing one of it's ends, thus setting a sone flying at a speed much higher that you can by just throwing it with your hand. You might imagine throwing pebbles at, say, a deer would not be very lethal and thus not very good for hunting, but the point is it doesn't need to be lethal - a good shot from a sling would cripple and slowdown it so you can come close and finish it with your spear or dagger or whatever you have, and if you manage to score a headshot you're looking at at least heavy concussion, at best an outright kill. And if you're hunting rabbits, it gets instantly lethal no matter where you hit. Come to think of it, hunting bow isn't much more lethal, and neither it should be, as crippling and slowing down is already enough.

Naturally people quickly found out that it was good enough for killing or crippling their fellow humans. It didn't even need that much of an upgrade compared to hunting bows vs war bows - just get a bigger pouch to shoot bigger pebbles at people. Maybe make a rope longer to rain stones on the enemy formation from larger distance - it's not like you need that accuracy to hit a block of men standing shoulder-to shoulder. Replace a pebble with a smaller lead bullet if you want it to shoot further, hit harder and potentially penetrate cloth and skin. And it worked fairly well.

From the Sone Age till early Medieval slings were actually dominating long ranged combat for two simple reasons: cheapness and ease of training. A bow requires good-quality wood and strings, more so if you want it to be used to shoot humans wearing at least cloth armor and basic helmets to death, and it requires fairly expensive arrows to shoot. Sling? You can make three for cheaper than a single arrow, and stones are basically free. Even if you decide to use more expensive ceramic stones or lead bullets, they're still dirt cheap compared to arrows, and are even compared to them in terms of range an lethality. To train a proper bowman you should start with a kid, while with a sling you can grab a buch of peasants, drill them for a few months and viola - you have a working unit of ranged fighters. Chances are you wouldn't even need to train them, as many peasants would learn sling techniques on their own to hunt some rabbits or to protect sheep form the wolves. Of course they wouldn't be as accurate, but who cares if you can field two dozens of slingers for the cost of a single archer, and replace them in no time if they happen to die?

Considering all this and adding the ease with which a sling could be carried it wouldn't take much money and effort to arm and train your spearmen and swordsmen with slings too. Romans did it with their legionaries, and it basically made their legionaries immune to enemy skirmishers.

Naturally there's a reason while people continued to train expensive bowmen despite the awesomeness of the sling. First off, slings are probably the least accurate military-grade ranged weapon (maybe second-least if you include throwing axes). Secondly, they lack in damage department: while it's true that blunt damage gives zero shits against most types of armor, hitting anything but a head aren't likely to knock the target down, and apart from lead balls at direct-fire range (15-30m) it would not stick into their body, impeding their movements or incapacitating them via pain shock and blood loss like arrows do. Lastly, a volley of rocks or lead bullets flying at you is almost invisible, while a volley of arrows or bolts bloating the sky is terrifyingly visible and prone to cause panic or at least disruptions in formation, and these disruptions became more and more valuable then the actual damage as warfare progressed.

One last bit about slings, is that they can throw things other than stones or bullets. Like vials of Greek Fire, or Chinese bombs, or grenades. In this last capacity they survived till the Great War, only falling out of use with the introduction of sub-munition grenades and auxiliary grenade launchers.

Fantasy and traditional games tend to ignore slings or pretend they don't exist. When they are included they are usually portrayed poorly and have close to unusable (or straight-up unusable) stats in the crunch. The only time you would see them used is in the hands of a DnD cleric when they were forbidden from using sharp weapons, or if some race (Haflings are the most common pick) get a sizeable bonus to damage or accuracy with it.

Medieval Weaponry
Melee Weapons:	Battleaxe - Dagger - Lance - Mace - Club Pole-arm - Spear - Sword - Warhammer
Ranged Weapons:	Blowgun - Bows and Arrows - Cannon Crossbow - Firearm - Rocket - Shuriken - Sling - Incendiary Weapons - Artillery
Armor:	Armor - Fantasy Armor - Helmet - Pauldron - Shield