I love a good theory. A good theory - however misguided - will always tell you something, if only about the author. It doesn't have to be right, even when theories are right they are only right until the next one comes along. But it might make you think in ways you hadn't thought before, question things you otherwise took for granted. With that in mind I thought I'd post a few links to some of the more interesting theories out there.
The first is one you probably haven't heard of - devised by a Russian of the name of Nikolai Morozov and expounded by Anatoly Fomenko it argues that we have measured the duration of history all wrong. In essence, it's only been roughly a thousand years since the time of Christ, and many of the antiquities and monuments of Classical Architecture are only a few hundreds of years old. Various devices are used to explain this, one of the more compelling of which is argument that "single events from other time periods that have been recorded multiple times from different perspectives, and thus mistaken for different events occurring at different times."
Note that the theories principal adherents tend to be mathematicians, or people somehow involved in the mathematical sciences. Garry Kasparov, the Russian chess champion, is one such adherent.
Link: The New Chronology