Far Realm
The Far Realm is one of the more obscure planes of Dungeons & Dragons, and basically amounts to the obligatory "Cthulhu Mythos Dimension" for Lovecraftian horrors. It is formally considered an invention of 3rd edition, although its roots lie in obscure D&D lore from the very beginning, in particular the adventure "The Gates of Firestorm Peak", which actually named the plane. 3rd edition merely expanded on the details most highly, which, combined with the fact it isn't formally placed on the Great Wheel, is why most consider it a 3e invention.
The Far Realm is generally described as, or believed to be, the origin-point for all aberrations.
Ironically, the Far Realm was about the only major plane to survive completely unchanged when 4th edition switched from the Great Wheel to the World Axis.