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alternatehistory.com
The role-playing game I love, GURPS, comes with its own default setting, GURPS Infinite Worlds. The assumption is that there are parallel universes, each with its own version of our Earth. They are all different, because some event changed history.
Some of the information here is in the standard format for GURPS Infinite Worlds, however I hope that readers who love alternate history and what-if ideas will enjoy reading this anyway.
So just go on!