Instapundit is talking about alternate history. No not revisionism that everyone hates, I mean the science fiction kind where something important occurred in the past happened differently and the world is changed. One notable set of short stories is the Lord Darcy collection written by Randall Garrett. In those stories the Plantagenett line of British kings (like Richard the Lionheart) continued unbroken and came to rule most of the world.
Another common theme that comes up is "what if the Germans had won WWII". An interesting short story I read on the subject had the Germans taking over colonial rule in India. Lets just say Ghandi did not fare as well against them.