Changed dungeon exits so that they have a minimum 15% chance of exiting to the Overworld if a dungeon's root dimension isn't the Overworld. Also made a minor change to the existing special case for the Nether - the minimum 20% chance only applies if the root dimension isn't the Nether.
DimDoors
Dimensional Doors for Minecraft Adds several items that allow the player to move between and create dimensions
see the MC forum topic for details