I'd also recommend Mystcraft. If you've ever played Myst or Riven you'll understand how it all works pretty quickly, but a quick rundown is that you can create special books that describe an Age (a dimension) and then you can create books to travel back and forth between them. You can customize what sorts of biomes you'll find, weather, day/night cycles, colors of the sky and water and grass (I believe, it's been awhile), all sorts of stuff, plus create randomized dimensions as well. You can find out more on the mod's thread and homepage.
ADVENT OF ASCENSION!!!!!! it adds 22 dimensions, more then 30 bosses, more then 320 mobs, more then 500 armor, weapons, bows, guns, cannons, riffles and more then 1000 new sound effects and music to the game!!! you have to get it!!!
or orespawn with mobzilla and other big bosses and 5 dimensions
or mystcraft you can make ur own cutsom dimensions
or any dimension mod, you can have dimensions of EVERY block