The Dark Tower
The Stephen King universe lends itself to world-hopping scenarios; thus, characters from different realities can encounter each other. Players can originate in many realities, including worlds very similar to our own modern day, as well as other realities like Mid-World (the world of the Dark Tower series). The collection includes rules on character creation, a book of 60+ inhuman creatures, two books detailing magic and psychic abilities, and a collection of character templates.
It can be downloaded here.
Based on the book series of the same name, the premise is fairly simple but full of history. We won’t get into too much detail because the books are bastard complicated so we’ll try to keep it relevant to anything that would be useful to a GM.
All-World is a parallel reality that is connected to all other realities (in practice, this allows Stephen King to connect many of his novels together and even draw in characters or their altnerate versions however he sees fit). This is made possible by being the home world of the Dark Tower, the lynch pin of all reality. All-World is an unstable realm (in the parlance of the novels, it has “moved on”) because reality has been weakened here; after a cataclysmic war in which people sought to control the Dark Tower and bend reality to their will, the beams supporting the tower have either weakened or collapsed. As a result, space and time are not consistent in All-World, dangerous reality fissures called “thinnies” have popped up everywhere, and demons have been released from the primordial “space between spaces.” Should the tower collapse completely, the world will be destroyed and be consumed by primordial demons in the chaos that follows. Hmm, where have I heard that before...
But demons and reality fissures aren’t the only dangers in All-World. All of the ancient technology of the Imperium (not that one) has not only been forgotten, but many of the surviving machines have gone homicidally insane. There’s also various mutants left over from the war, as well as a race of beast-people called Can-toi who work for the Crimson King. And then you have more mundane threats like bandits, giant cyborg bears, and lobstrocities that go “Did-a-chick? Dad-a-chum?”
Now, a quick word on dimension hopping: there exist many doors on All-World, rarely in places that are easy to find. You won’t always know where they lead, as they can even go to different time periods in the same world, or to near-identical versions of said world. Most doors will be found in Castle Discordia, deep within the Crimson King’s territory in End-World. Some can be found in the wild, especially the traveling “Unfound” door that requires the exceedingly dangerous artifact known as “Black 13” to open. But it is also the only door that will take the user anywhere they want, rather than a preset destination. The Unfound door is also the only way to enter the Dark Tower itself, requiring one of the artifacts of King Arthur to open.