There have been reform movements within the church for practically as long as it has existed. For example, you can see movements such as the Montanists, in the second and third centuries, as (partly) calls for ethical reform. The same thing underlay the Donatist movement, which split off from the Catholic Church in the early fourth century and became the dominant form of Christianity in north Africa for a while. Pelagianism, too, was inspired by the supposed laxity and frivolity of the new Christian congregations following the proscription of paganism, and was an attempt to make everyone live like monks.
All of these movements were condemned as heretical and ultimately failed. There were more successful reform movements in the Middle Ages, partly because there tended to be so many things needing reform in that period! These happened both within the church as a whole and within particular orders, such as the monastic orders; and in each case there tended to be cycles of reform followed by a gradual relaxation of standards, followed by a new period of reform, and so on. Some of the most important reforms in monasticism included the Cluniac movement in the eleventh century and the Cistercians in the twelfth and thirteenth. In the church as a whole, one of the biggest periods of reform was in the eleventh century, under Pope Leo IX and then, even more importantly, Pope Gregory VII. These popes and their advisers devoted huge energy to trying to stamp out corruption, especially among the priesthood and in the monasteries, and practices such as simony and the inheritance of ecclesiastical positions. Peter Damian was an important figure in this movement. In the twelfth century, Bernard of Clairvaux was something of a reformer, calling for high moral standards at all levels of the church. And later, Pope Innocent III brought in many reforms too, culminating in the fourth Lateran council. It's not hard to find plenty of contemporary criticism of standards in the church; just read the General Prologue to Chaucer's Canterbury tales. Of course, there are people in every generation who think things are worse than they ever have been, so one shouldn't credit the woeful descriptions one finds throughout church history too much.
That period also saw reform movements that were condemned. These included Petrobrusianism in the early twelfth century (a movement associated with Peter de Bruis and also, perhaps, Henry of Lausanne) and, arguably, Catharism, which was partly ethically motivated. Later in the late twelfth and the thirteenth centuries, the Waldenses also called for reform of the church but were suppressed (because they disobeyed the church authorities by preaching without permission). The most important figures like this from the viewpoint of the later Reformation were John Wycliffe and Jan Hus in the fourteenth and early fifteenth centuries. They called not simply for reform of standards but for reform of the ecclesiastical system itself, with Wycliffe going so far as to call cardinals servants of the devil.
So there had always been attempts at reformation within the church; some had been successful and some had not, for various reasons. When Luther nailed his theses to the church door he was simply joining that tradition. What was different about subsequent events was that the reform movement spilled out of the church and resulted in new churches being set up, which, unlike the earlier Donatists and Hussites, were successful and flourished. That hadn't been Luther's original intention at all. Also, much of the reform movement remained within the Catholic Church, resulting in the council of Trent, the establishment of the Jesuits, and so on. These are traditionally counted as part of the "counter-Reformation", as if they were simply a response to the Reformation, but really it's more accurate to see both the Reformation and the counter-Reformation as parts of a single movement of reform during that period. It's just that some of it resulted in schism, while some of it was contained within the Catholic Church.
The direct influence of earlier reformers upon Luther and his contemporaries is hard to say (and it's not my period!). Certainly, when the Reformation began and the first Protestants split off from the Catholic Church, remnants of earlier reform movements - notably the Hussites, the Moravians, and the Waldenses - merged with them, so their ideas were influential there. The ideas of Wycliffe and Hus, in particular, were certainly influential on the Reformers. But like all reformers, Luther and the others were primarily reacting to what they believed to be problems of their own day. When Luther called for a debate about the practice and theory of indulgences, he did so simply because he thought it was something worth discussing, not because he wanted to be another Wycliffe.
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