Other alternate history titles:
Ward Moore's Bring the Jubilee...it may be the South Wins, but Winston Churchill and Mackinlay Kantor look sick compared to this...but put it up against Terry Bisson's Fire on the Mountain, where we all win the right kind of Civil War, thanks to John Brown!
Mona Clee's Branch Point, about an alternate ending to the Cuban Missile Crisis vs. Brendan DuBois's Remembrance Day.
L. Sprague de Camp, Lest Darkness Fall, much like Turtledove's Guns of the South, except the help from the future goes to the Goths fighting Justinian.
Melissa Scott, A Choice of Destinies, Alexander the Great vs. Rome, then Carthage.
Turtledove is a truly godawful writer who cranks'em out like sausages. But I think his genuine best is Agent of Byzantium, because his field really was Byzantine history, and I think it shows.