Ultimate Conspiracy: I just read "The DaVinci Code" which, to grossly summarize, is a fictional thriller using the factual (or at least historically verified) backdrop of an alternate explanation of Christianity. TDC is written by the poor man's Umberto Eco, Dan Brown. The book's prose is fairly average, and the storytelling, completely linear with almost no insight into the characters' minds, but that's not why it's worth reading (or at least skimming). Now, I had heard of the Knights Templar, the Rosicrucians, and the theory that Jesus was married to Mary Magdalene before. I had also read about semiotics before (Eco being one of the most popular proponents of same).
But Brown gives us an abridged version of how Christianity was subverted by the Papacy and certain monarchs over the years to the point now where it hardly resembles its "original" condition, and along the way points to some wonderful markers, which, if you weren't looking, you'd miss. The most famous may be DaVinci's "Last Supper". The symbology contained in that painting is quite extraordinary, and shows his clear disdain for the Catholic Church and its alleged abuses. Anyway, I can't do it all justice, but suffice to say, if you want a conspiracy, look no further.