There's huges difference between "low class" in medieval armies between the Early Middle-Ages, Classical Middle-Ages, and Late Middle Ages.
In the first case, theoritically every free man have to participe to plaids.
Bow and arrows for the less fortunate, for the others a spear or a scramasax, sometimes an axe, an angon or a shield (basically everything that have an use in everyday life)
Even semi-free or servile population could participe in warfare, restricted as frankish lidi or plainly under the orders of their patrons as visigothic or burgondian slaves, sometimes lombardian.
Erwig's law mention that for every man (Goth or Roman) shall bring 10 slaves to war, equipped with weapons (Be wary, even at this period, the king complained that the ratio wasn't even 1:20)
It changes with Carolingians, mostly due to the military technology advance, that ask for cavalry-based armies.; but is maintained in different places as Anglo-Saxon fyrd where the military service is still due.
For the most part of Classical Middle Ages (up to XIIIth century, roughly), the peasants are imbelle : theorically, they doesn't fight, or don't play a great role.
In reality, you have the participation of pedites, habitatores more or less obligatories in the levies, critically for general war or to defend the land : you don't have that of a distinction between building/repairing a wall or a castle, and participating to defense for the poor peasants (roughly 6/10 of the population)
The weaponry is rudimentary and often based on agricultural tools, but influenced the devellopement of war weapons : flail, war scythe or bills are obvious exemples (from flail, scythe and billhook).
Bow and spears are as well present, if relativly less.
Places as Spain have a more important place in warfare for peasantry, but still place themselves in a general tendency : pedones are issued from cities, and caballeros villanos highlight a popular cavalery that must have been close to their Frankish equivalent that evolved up to forming the knight (milites) classes, but had a role more similar to italian communal cavalry.
Peasants (both poor and more wealthy) in this period are essentially forming a more or less passive infantry : contrary to what is broadly said, they weren't "cannonfood" though.
The pitched battles weren't admittedly their place, and they were more skirmishers and archers than expected to fight more directly as knights (but as well middle-class or high-class peasants whom equipement was more elaborated), because (among other reasons) they were expected to not being able to hold well formations, and their role was essentially auxiliary (both in pitched battle and sieges)
It changes with the mid-XIIth century.
With the rise of an urban and peasant infantry with a greater role, poor peasantry place declined, while other parts of peasantry maintained or grew on their side.
Infantry bodies, as mercenaries, evolved often from poor peasantry; but they formed their own entity at this moment. The evolution, aforementioned, of agricultural tools to weapons could be a symptom.
Peasants can still be raised in levies (especially in lands lacking men such as Jerusalem, Scotland or Switzerland), but the equipement of the poorer fighters is interesting.
It doesn't means poor peasantry cease to be integrated, but the specialisation of the roles they hold before (such as archery in England) is more about selecting the fighters (Simon de Monfort iun 1264 didn't proposed anything different) in a general trend were cities and selection takes a greater importance, in par with mercenaries (that are after all, a consequence of the former).
As for the equipment, revolts as Jacqueries point that the use of agricultural tools, as well equipment taken on fallen fighters, was probably the rule.