Let's get physical, and by that of course we mean theoretically physical.
An alternate timeline and an alternate universe are not separate entities. An alternate timeline encompasses an alternate universe. An alternate timeline takes place in an alternate universe. The timeline variations can be small like one where Gwen Stacy gets bit by the radioactive spider or one where the prime species on Earth are apes.
They are not different concepts.
With that explanation given, comics tend to provide the best example and theory on the many worlds idea. For a start it's fictional, so the variations of reality are only limited by the human imagination and – one supposes – the strength of a given pharmaceutical.
The other reason comics keep going back to the alt-timeline well is for its infinite potential for stories: without having to explain history or motives, you can explore an established character in a new exciting environment.
They're popular too. The Dark Knight Returns is set in an alternative timeline and is considered one of the best comics ever made.
We're not necessarily looking at the best, though some quality material does appear, we're looking at weird or just completely out-there stories that make the reader look back at what they've just digested, shrug their shoulders and go 'huh'.
10. Superman: Red Son Everyone Is Russian
Superman is a super man – he's got super speed, super strength and a super belief in laissez-faire economics.
There's a theme to a lot of alternate Superman timelines that generally follows the rule of: Kal-El rocket lands in X = Y Superman, where Y equals whatever area Superman happened to land.
Superman may not really be American, but he has come to represent the US both within the DC universe and in the real, less colourful world.
In 2003 readers were invited to visit a world where Earth time was a few hours off from our real time; this very small modification leaves the ship carrying Superman to Earth landing in the Ukraine rather than Kansas.
The story of Red Son begins there and then gets really, really weird.
Superman becomes a tool (more so than usual) for the Stalinist Soviet Union, Lex Luthor becomes something of an American hero and Batman gets a cool ushanka.
The weirdness is moderate, until the end when it turns out it was Krypton all along. Spoilers, by the way.