The Archive of Alternate Endings by Lindsey Drager is my absolute favorite.

honorable mentions:

• Slumberland by Paul 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