It seems a bit odd to eulogize a man who has been gone for nearly 30 years, but it seemed a fitting way to note the centenary of Francis Schaeffer's birth.
Schaeffer was arguably one of the most influential Christian apologists of the 20th century (along with C. S. Lewis). Although I read several books by Lewis before ever cracking open a volume of Schaeffer, it was the latter that has had a more profound effect on my worldview—especially his books The God Who is There and Escape from Reason, and to a lesser extent Art and the Bible and Pollution and the Death of Man. It was through Schaeffer that I first discovered the value of presuppositional apologetics, and he may have been my first few baby steps toward Reformed theology. And, although he was not a direct influence on my own views, in the late 1970s he was one of the earliest Evangelical pro-life voices, whose influence made opposition to abortion a Protestant, not merely a Roman Catholic, concern.
Also, as I have noted previously in my lightning review of The Church at the End of the 20th Century, although some of his work feels dated today—much of his ministry during his most productive period was with the hippie counterculture of the 60s and 70s—many of the worldview issues he raised back then have come full circle. With the rise of draconian campus speech codes and pointless activism like the recent Occupy movement, perhaps we're in need of a fresh look at Francis Schaeffer.