Outertale: Undertale but in space, duh.
Horrortale: Undertale as a horror story.
Mafiatale: Undertale as a cowboy story.
Glitchtale: Undertale as an anime story.
Underpants: Undertale as a fun sitcom.
Underfresh: Undertale for the hippies.
Fanfictales: Generic undertale worlds.
Batter's World: The setting of the game 'OFF' when the batter goes on a quest to purify the world. It was revealed to be a wacky test world created by W.D. gaster.
* Notes: Crossover worlds don't exist. *
--------------------------------------------------------
There are many minor undertale worlds.
--------------------------------------------------------