The Mammoth Book of Alternate Histories edited by Ian Watson and Ian Whates. Here's a quick collection of thoughts, largely off the cuff, about the first few stories that I've read so far:
"The Raft of the Titanic" by James Morrow, pp. 1-29.
A peculiar story to start the collection with, Morrow springs a Utopia from the disaster of the Titanic by using the powers of hindsight and retrospect which alternate histories are so reliant on to suggest a possible method by which all of the passengers of the luxury line might have survived. The narrative is light and whimsical, deliberately and explicitly avoiding the potential darkness that could be associated with being castaway on a raft ala Coleridge who is quoted in the story, or Yann Martel's Life of Pi, indeed there are echoes of the latter's darker ending in the humorously portrayed cannibalism of less well liked members of the crew, and the raft inhabitant's reactions to news of wider world without them. Interestingly, this is one of those alternate histories like Philip Roth's The Plot Against America, where despite the change in the historical record the course of major events plough on regardless.
"Sidewinders" by Ken Macleod, pp. 30-42.
So far in my reading of alternate histories, the number which could be called true sf is relatively small. The purest example are probably the time machine alternates like Ray Bradbury's "A Sound of Thunder", and novels such as Ward Moore's Bring the Jubilee and Stephen Baxter's The Time Ships. Macleod's story is one of these, but rather than time travel the science fiction here is based on dimensional hopping more akin to the TV show Sliders than anything else. It's a lively and exciting story which does a lot with the small space it has, indeed it successfully creates the idea of a wider Universe in which I could imagine a novel, or even a series of novels, being set.
This one was a fascinating Jewish alternate history. One of the things that made it so interesting was actually the account of the historical record from the year 0 to what we would consider 1000AD. Narrated by an immortal who has survived from the time of Christ and yet remains anonymous and vaguely in the background of his own story, the historical divergence here is the failure of Christian Roman Emperor Constantine to secure his empire and thus to create a dominant Christian religion in Europe. Up until this moment Newman and Byrne tell one of the most engaging and vivid accounts of the life and Crucifixion of Christ that I've encountered (admittedly religious fiction isn't an area I've often explored so I have few points of comparison). After the divergence the narrative remains interesting, with an alternate version of Charlemagne founding a Jewish rather than Christian dynasty. Again humour is strong in the story, though mostly black, and I especially like the references to a Britain - 'a cold, miserable, wet, piss-sodden island that I don't recommend you ever visit.'
"Hush My Mouth" by Suzette Haden Elgin, pp. 83-90.
There are a heck of a lot of "if the South had won" American Civil War alternate histories. Elgin twists the norm by suggesting that if both the North and the South had refused Blacks the right to fight in their armies than neither would have edge enough to win and after a number of years the war would just fall apart as the two Disunited States crumble. Rising up the former slaves take control of the exhausted and ravaged South and expel the white men into the North. Dubbing their land "New Africa", Elgin explains that race isn't a factor in human stupidity and through an excess of pride the various groups of New Africans cannot agree on a common language with which to govern their new nation, each preferring the language of their old African tribe. The linguistic twist is typical of Elgin, whose Native Tongue I wrote on as part of my Masters Dissertation, and her suggested solution to the quandary is an interesting one. Overall an effective, if short piece.