DC comics, blessed with three of the most iconic super-heroes in the history of the medium - The "Trinity" of Batman, Superman, and Wonder Woman - created an imprint in the late 80s that would feature "imaginary" stories set in alternate timelines or realities. These stories would be released under the imprint of Elseworlds.
Gotham by Gaslight
In 1989, the first of the Elseworlds story, Gotham by Gaslight, was released. Featuring Batman, but set in Victorian times, the caped crusader has to hunt down Jack the Ripper who has come to Gotham to continue his murder spree.
Although not officially branded with the Elseworlds logo, Gotham by Gaslight is considered the first of the Elseworlds books.
Superman: Red Son
One of the most lauded and critically acclaimed Elseworlds story was Superman: Red Son, which wonders what would've happened if the baby Kryptonian had crash landed in a farm collective in the Soviet Union instead of Kansas.
The three part story was written by well known Scottish scribe Mark Millar who wrote Kick-Ass, Most Wanted (both turned into movies) and Civil War (which is being used as the blueprint for the upcoming Captain America: Civil War.)
There are many Elseworlds worth checking out...Batman during the American Civil War or as a Vampire fighting Dracula...
Superman growing up in Arthurian England and forging the sword Excalibur...
Robin in feudal Japan...
The Legion of Superheroes in their own Wizard of Oz story...
Superman and Wonder Woman in a story featuring Greek mythology and Nazis and set in the future. :D
There's something for everyone. Check them out.