Alternative history straddles speculative fiction and the practice of asking counter-factual questions in the academic study of history. I believe that alternative history can be entertaining, but also useful as a tool to help separate the contingent from the necessary, the underlying causes of real historical events from the accidental ones.
I am the FAQ maintainer for (and also a regular contributor to) the Usenet newsgroup
soc.history.what-if (shwi), the subject of which is alternative history. Shwi
has a nice mix of discussion, alternative timelines,
narrative episodes and essays. It's a high traffic newsgroup, so
while the signal to noise ratio is pretty good there's always quite a
bit of rubbish floating around. Ignore it; the rest really is worthwhile. There are some very knowledgeable people contributing, the stories are well-written, and scarcely a week passes without someone saying something quite thought provoking. There is also a strong community spirit, which makes regular postings a pleasure.
For more information on alternative history and shwi, I can do worse than point to the official FAQ, a link to which is here:
soc.history.what-if Frequently Asked Questions
The rest of this page contains some work by master timeline writers of soc.history.what-if. My own work needs a thorough overhaul before it is presented here.