As every schoolchild ought to know, on October 31, 1517 Martin Luther wrote to Albert, Archbishop of Mainz and Magdeburg, protesting against the sale of indulgences in the bishopric. What has caught the imagination of posterity is Luther also nailing the 95 Theses to the great door of the Castle Church in Wittenberg.
Briefly, indulgences were among the most obvious of the many corruptions the Church was suffering from: documents produced by the Papal Curia that, if bought, supposedly allowed people not to suffer for their sins in Purgatory. You could buy them for yourself or others.
So, errm, what has that to do with the modern world. This is not even the anniversary. Well, there are new indulgences around. They are called carbon offsets. Here is a longish piece about the whole phenomenon.