Along with the aforementioned goofy Silver Age stories that saw Batman grappling with his alternate universe doppelgangers and Superman imagining what having kids would be like, DC dipped their toes back into the multiverse again with their Elseworlds line of comics. Designed to draw in creators who might not have worked with them otherwise, or were just itching to tell some out-of-continuity stories, the Elseworlds books created a whole slew of new different Earths that let writers and artists go off in all kinds of new, weird directions. There was Gotham By Gaslight, drawn by Hellboy creator Mike Mignola, which shifted the timeline back a notch so Bruce Wayne was facing up against Jack The Ripper in a late 1800s Gotham City (now designated Earth-19). There was Red Son, wherein Superman crashed not in Kansas but Soviet-era Russia, being raised as a weapon and propaganda tool for the USSR (Earth-30, fact fans). Most famous of all was probably Kingdom Come, where the superheroes got overtaken by a new generation of more bloodthirsty metahumans, leading to a huge civil war (Earth-22, we think). That's, like, a whole sub-world of crazy multiverses.
Tom Baker is the Comics Editor at WhatCulture! He's heard all the Doctor Who jokes, but not many about Randall and Hopkirk. He also blogs at http://communibearsilostate.wordpress.com/