The Archive of Alternate Endings by Lindsey Drager is my absolute favorite.
honorable mentions:
Slumberland by Pauly Beatty
A Confederacy of Dunces by John Kennedy Toole
Slaughterhouse-Five by Kurt Vonnegut
Kafka on the Shore by Haruki Murakami
The Thought Gang by Tibor Fischer
The Things They Carried by Tim O'Brien
The Cider House Rules by John Irving