A book I've had for years (but haven't gotten around to reading yet, so I can't comment on how good it actually is)
http://en.wikipedia.org/wiki/The_Years_of_Rice_and_Salt
A trilogy I have read involving an accidental opening to a parallel universe where the Neanderthals survived and the modern humans didn't:
http://en.wikipedia.org/wiki/The_Neanderthal_Parallax
And you might or might not like Pastwatch from Orson Scott Card:
http://en.wikipedia.org/wiki/Pastwat...opher_Columbus