Leather Armor

Leather armor, despite the popular misconception, is not soft and supple like the leather used to make a ranger's boots or a druid's robe. That kind of leather offers no better protection than common clothing. Leather armor is actually strong and stiff, having been hardened in boiling oil and then stretched over a wooden or stone model of a man's or woman's chest. The resulting breastplate and shoulder guards are combined with a tunic or kirtle and, in colder climes, leggings of wool or soft leather.
This is the most common form of "modern" armor. The materials (leather and oil) are readily available in all civilized lands. Only the technique for boiling and shaping the leather is necessary, and this is not a difficult feat for a leatherworker.
This armor is both inexpensive and durable. While the leather is extremely stiff, it is never fashioned into anything larger than a breastplate, which keeps restriction of movement to a minimum. Naturally, this arrangement means leather armor affords no protection to the joints, but this is true of most types of armor, and is a challenge all civilized races have been attempting to overcome since wars began.
In severe combat situations, leather armor may need to be replaced weekly. However, the armor is easily cleaned, reasonably unaffected by weather, and resists all but the severest of abrasions. This means a good set of leather can be worn daily for many months without need for replacement. Many retired warriors and middle-class militia have a set of leather armor stored away that they take out and polish at least annually.
Because raw leather comes from a byproduct of medieval daily life (i.e., eating beef and other meats), in civilized societies leather armor is very common. Even rural communities have little trouble manufacturing leather armor for the troops within a few days.
Cows are not the sole source of hide for tanning. Horses, sheep, and camels can be used just as easily. In short, the creature must have a thicker skin than that of a normal man, but not quite as thick as that of an elephant or bear. The skins of these well-protected creatures, when tanned, becomes hide armor (q.v.).
Irregular human forces (e.g., militia and levies, freemen, commoners above peasant level, barbarians, light infantry, and marines) are the primary users of leather armor, for the following reasons: A) leather armor is drastically cheaper and more readily available than metal armor; B) the armor can be worn for long periods of time without leading to increased fatigue or disease; C) the method of production is so simple that many armies can make new sets of armor from cattle seized in newly raided territories, often scant days before meeting the defenders in battle; and D) they can be stockpiled for years without the need for the excessive maintenance required by metal armors that are prone to rust.
Another benefit of leather armor, much extolled by those of a more disreputable persuasion, is the ease with which it can be silenced. While leather armor is not typically noisy, its buckles and fasteners tend to rattle and clink. Unlike metal armor, leather armor is easily muffled by clothing and as such makes little sound during normal movement. Additional layers of clothing further dampen sounds made by the wearer while hiding. More rogues than can be counted owe their lives to muffled leather armor.
The freedom of movement, adequate general protection, lack of noise, high availability, and low price make this the armor of choice for the general human population.