This mod removes the biome clustering by temperature in 1.7 and instead distributes biomes randomly, as they were in 1.6.4 and earlier, so you can have hot and cold biomes next to each other (biome size and terrain generation is otherwise default). In addition, a new world type is added (Random Biomes, using ID 15), so you can still play default/large biomes/amplified worlds in the modded game without worrying about messing them up.
Select Random Biomes as the world type in the world options menu:
This is the result, looking down at spawn, using the seed shown above (-123775873255737467):
From left to right, clockwise, you see a mega taiga, taiga, jungle, ice plains, plains, and swamp.
Now, here is a larger scale rendering of the world (a bit larger than a level-3 map, 1024 blocks square); the number of biomes in 1.7 means that you’ll still have to explore a bit to find all the biomes
Also, as an optional part of this mod, is a cave/ravine/mineshaft mod which modifies cave generation to get actual cave systems and not random caves all over the place, makes ravines more variable/interesting, and a more uniform distribution of abandoned mineshafts (no decrease in spawn rate close to the origin and a minimum spacing between complexes, with a lower overall chance).
Here is a comparison between normal cave generation (left) and modified (right); note that while cave systems are larger and denser, there is also more empty space between them, with no small caves scattered around (i.e. a minimum size was set):
In addition, caves and ravines were made to not cut through the surface (dirt/grass layer, similar to caves in desert/mesa biomes), except for rare small cave openings, one per cave system (more if there is a compound cave system), note that these caves spiral all the way down to bedrock:
Random Biomes Caves: You MUST install the Random Biomes part for this to work, or the game will crash; the changes also only affect the Random Biomes world type.