Climate control allows you control climate zones, ocean sizes, biome frequencies, and continent sizes. Make Minecraft worlds into Yourcraft worlds!
Set climate zones to smaller, more manageable sizes so you can find a variety of biomes more easily.
Prevent hot biome-cold biome transitions, if you want, or mix all the biomes together.
Change the frequencies of common biomes and climate zones to occur as often or as rarely as you want.
Restore true oceans to 1.7, or keep a universal continent with large lakes.
Create a variety of land sizes, from giant continents tens of thousands of blocks across to islands that fit on a single map.
Make climate bands to mimic an earthlike world.
Set initial spawn area to any land area anywhere.
Link to more detailed discussion of landmass control towards the bottom of the MC Forum thread.