Elder of Ziyon notes that today is the 900th Yahrzeit (anniversary of the death) of Rashi, one of the great Jewish scholars of all time. His explanations of the Bible and the Talmud are essential to understanding the works. And when he wrote them, he didn't have a word processor. His work was absolutely monumental.
In his honor here's a link to a discussion I started in Mail-Jewish about Rashis Descendents (scroll down). Actually I just asked the question. Michael Gerver did the hard work. I also put the question to Rabbi Wein a few years ago after the screening of his movie about Rashi. Rabbi Wein estimated that 80% of Ashkenazi Jewry is descended from Rashi. Michael Gerver's calculations led him to believe that the percentage was somewhat higher.