One often hears that medieval peasants had to pay a certain percentage of their harvest and an unlimited amount of their labor to their landlords. This (along with so many other myths about the Middle Ages) is not true. They certainly owed produce and labor, but it wasn't unlimited.
This is because medieval peasants were not slaves, as I have discussed earlier. Indeed, in the early twelfth century even serfdom, being legally bound to one's "lord of the body," disappeared in France (see more here). But both serfs and free peasants owed rent, just like anybody who rents an apartment today owes rent.
The rents were generally a combination of money, of produce (like two bushels of wheat and a chicken each year), and labor (like having to work on the lord's land two days a week). Rents were fixed and were not supposed to be raised. You will note that they weren't a percentage of the crop; the only "percentage of the crop" in sight was tithes to the church, which many peasants did not pay at all because most villages out in the country did not have a church.
The labor dues were often the most valuable part of the rent to the landlord. Unless he planned to be out there with the plow and the ox himself, he needed to have people working his fields to grow the food for his household--and to grow food that he could sell at market (thus generating cash to buy horses, silk, spices, jewelry, and all the other things aristocrats felt they needed). If every tenant household had to provide someone to come work on the lord's land (his demesne) at least one day a week, the necessary work got done.
This labor in fact could be beneficial to the peasants, although they probably didn't appreciate this fact. The chief way it benefited them was to acquaint them with new, expensive technologies, like heavy mould-board plows, which they would have been (understandably) reluctant to try themselves, knowing that if they didn't work as promised then their families would starve. The landlords, however, could afford such experiments (as well as affording the equipment). Once peasants had seen that something worked, then they could adopt it themselves.
In addition, peasants could ride along with a lord when he took produce to market. They might only each have a small amount to sell, but by going with him, and his larger amount, they found it worth it.
But peasants hated labor dues. They would much rather be putting the effort into their own fields than their lord's. By the twelfth century, a lot of lords were finding it a total pain to enforce labor dues. The workers would show up late--with expanding cultivated land, the peasants might live miles away, and you couldn't expect them to leave home before dawn--leave early, and expect lunch. Many landlords "commuted" (as it was called) labor dues into an extra monetary payment. Then they could use the money to hire day laborers, who would not get paid if they didn't show up on time and work hard. Young men could start saving up money by working as paid laborers.
Many a lord who owned unused, woodsy or swampy land would establish "new towns" (actually new villages), hoping to attract peasants (that is, other lords' peasants) to come work on land that hadn't been producing anything before, peasants who would pay rent. In these new towns, the peasants would not owe labor dues, generally just money rents and maybe a little produce.
In the expanding economy of the High Middle Ages, the cost of hiring labor kept rising, and it became harder and harder to hire good workers on the cheap. Rents, however, were fixed as they had always been. What had once been a good deal for landlords no longer was. As a result, in the thirteenth century landlords stopped commuting labor dues and tried, generally with minimal success, to reinstate them.