"Our world is still secretly ruled by the race descended from Ikhnaton"... or at least, that's what Philip K. Dick says in his paranoid masterpiece Valis. In Dick's world, Ikhnaton is supposedly an alien who "emanated from a planet in the star-system Sirius". In the world where sane people live, however, Ikhnaton is simply a variant spelling of Akhenaten -- who was an ancient Egyptian Pharaoh of the 18th dynasty.
Akhenaten's religious revolution only lasted as long as his own reign, but that fact hasn't deterred the theorists. Although the Aten had almost nothing in common with the "one God" of the Israelites, many people assume it was one and the same... Akhenaten is often identified as Joseph's dreaming Pharaoh from the Book of Genesis, and he has even been credited as the author of Psalm 104!
A lot of the mystique surrounding Akhenaten stems from the fact that for three thousand years very little was known about him. During his reign he moved the whole Egyptian court from Thebes to an obscure place named Amarna... which was lost when his successors moved the court back to Thebes, and was only rediscovered in the 19th century. Amarna is an anomaly in the history of Egypt, quite different from anything that came before or after. Its oddly stylized artistic representations, which account in part for the "Akhenaten was an alien" theories, were boosted into popular awareness by the notorious Amarna Princess statue -- the most ambitious of the forgeries produced by the Greenhalgh family of "artful codgers".