My favorite is what would have happened if the English pirate [1] Francis Drake had captured Cartagena Columbia. He had several ships and about 5000 marines for his third attempt and almost captured the city. Imagine if England and not Spain had the gold and jewels flowing from Central and South America. History would have been very different - and that last battle could have gone either way.
[1] Both times I have been in Cartagena, I heard someone refer to Sir Francis Drake as a pirate. Personally, I have nothing against the guy :-)