When I first encountered Alternative History (also known as Alternate History, or Althis, or AH), it was an obscure interest, scarcely a recognized genre. Now it threatens to engulf science fiction, and indeed historical fiction. You should have no trouble finding resources online. Let me now just suggest a few:
Inevitably, there is also an Alternate History Wiki. Do not neglect the the Alternative History Web Ring, of which this site is a member. The menu for that is below.
Why do I prefer "Alternative History" to "Alternate History"? Because the latter suggests just two possibilities, as in "alternating current." "Alternative History" is also sometimes used to mean "Revisionist History." This practice should be ruthlessly suppressed.
Airships often appear in Alternative History.
This is because they are a Fortean technology that signifies a timeline
is diverging from the Main Sequence, as we saw in the first half of our own 20th century.
Year dates refer to the time of publication or posting. They are all in the Gregorian reckoning: no fooling around.
The Red Napoleon
(Floyd Gibbons describes the near conquest of the world by the Reds.)
(Harry Turtledove tells what would have happened had the Munich Conference failed.)
Count Your Blessings
(An Alternative New Year's Day for 2009.)
Toward the Court of the Crimson King
(A macrohistorical meditation on At the Courts of the Crimson Kings, the second volume of S. M. Stirling's AH neo-pulp series, this time on Mars.)
President John McCain
(That's "President John McCain: January 2001 to January 2009.")
The Sky People
(S. M. Stirling brings neo-pulp AH to Venus.)
There was no 2007. It's a long story.
The Gray Havens
(No, The Lord of the Rings is not history, but it lends itself to alternative-history treatment nonetheless. This is my own fan fiction.)
The World Hitler Never Made
(Gavriel D. Rosenfeld does a remarkably comprehensive survey of the allohistory about Hitler and the Third Reich.)
The Years of Rice and Salt
[Kim Stanley Robinson imagines the post-medieval world without the West: possibly the best AH in OTL.]
[This is a Thursday Next novel, by the oddly frictive Jasper Fforde. The time travel is the least of it.]
The Plot Against America
[Philip Roth's apocalyptic account of a counterfactual Lindbergh Administration.]
In the Presence of Mine Enemies
(Harry Turtledove presents a Soviet-style crisis for the Third Reich.)
For Want of a Nail: If Burgoyne Had Won at Saratoga
(Robert Sobel comes as close as anyone has to writing counterfactual history that looks like the real thing.)
The Man in the High Castle
(The Iliad and the Odyssey of Alternative History.)
(So, you wanted a sustainable world, did you?)
The Great Disappointment of 1844
(Do not expect too much of the end of the world.)
(An Alternative Obituary.)
(This collection of "Alternatives and Counterfactuals," edited by Niall Ferguson, is the real deal when it comes to serious alternative history.)
The Pity of War
(A review of Niall Ferguson's wrong-headed but fascinating study of the First World War and the possible historical alternatives.)
What If the Second Temple Had Survived AD 70?
(The answer is that there would be no Israel in the 20th century. It's a long story.)
Cthuluism and the Cold War
(A Halloween piece.)
The Xin Dynasty of 1916?
(An ambitious general tried to establish a new dynasty in China after the Revolution of 1911. Here I consider what would have happened if he had succeeded.)
(This started as an alternative history of English spelling in the 20th century, but it shows every sign of getting out of hand. A disclaimer is in order: I am a member of an organization called the Simplified Spelling Society . The SSS really does promote spelling reform, but they are not to blame for Altscript.)
Time on My Hands
(A review of a novel by Peter Delacorte that shows how wonderful life would be if Ronald Reagan had never been president.)
(A review of Kim Newman's novel, based on the premise that Dracula had married the widowed Queen Victoria. This book makes it unnecessary to ever read another vampire story.)