This is a very simple but very useful mod. It allows you to pick another option in the world creation menu that forces the whole world to be of a certain biome type. This is especially useful for creating adventure or survival maps in a certain biome.
Picking, for example, a forest world, spawns you right in a forest and the whole world is one big forest!
There are many world types, including swamplands, jungle, forest, and ice mountains. Check out ice mountains!DownloadPlanet MCInstall Guide