I've always found alternate or alternative history fascinating. What this is is a literary genre where changes in the historical timeline are explored (hopefully with good research put in to make the new timeline plausible). Good alternate history timelines are a great way to explore historical concepts and the reasons behind historical processes. The most popular subject for alternate histories is what would have happened if Hitler won World War 2, there are a number of interesting books i have read over the years covering this and other alternate timelines.
One book i read years ago (unfortunately i can't remember the author or title) was set in the "present day" but in a very different world where the Turks had overrun Western Europe in the Middle Ages and the Maya had not been defeated, and indeed were by now travelling around in steam drive carts.
I found this website a while ago, in its fora are a number of great alternate timelines (as well as some nonsense to be fair). No sign yet though of the ultimate alternate history scenario, what if Ringo had been a really good drummer?