In the Gettysburg Trilogy, the Battle of Gettysburg went differently than in reality, with knock-on effects for the rest of the Civil War.
In 1945, Germany stays out of the War in the Pacific and the USA stays out of the War in Europe.
China Takes Over the World: In One Second After, China is one of the few nations to survive the EMP attack comparatively unscathed. The Chinese take over the Pacific Coast at the end of the novel, and are actually welcomed as heroes because they bring much needed food and medical supplies. Mexico is threatened by the Chinese military buildup in California, leading to rivalry.
Divided States of America: By the end of One Second After, the USA has split into several parts. China has taken over the West Coast, Mexico has grabbed a large chunk of the South West, the US Government controls the East Coast and the rest of the country is more or less up for grabs and in chaos.
Dyson Sphere: The Gamester Wars series has a Dyson Sphere setting that's still being built—the Precursors' ancient robots have been at it for millions of years and it's still only half-complete, because of its size. There's also a Ring World Planet and other stellar-scale objects in the same universe.