For those of you interested in alternate or esoteric histories of the world, this one is a doozie. If you've read the DaVinci Code, this is of that flavor. I love the history of the Crusades, and I find all this stuff tantalizing.
I don't know necessarily believe its true, but its a fascinating idea. This is a particularly well-written summary of a few of the major theories.
Mary Magdalene: The Grail Bearer