Bukkit

Everything is configured in the config.yml

It has the following layout:

Default:
  somespawntype: true
  Mob:
    Mobname:
      SpawnEveryX: 1
      MinSpawnAdd: 0
      MaxSpawnAdd: 0
      Health: 13
      MinLightLevel: 4
      MaxLightLevel: 9
    OtherMob:
      SpawnEveryX: 1
      MinSpawnAdd: 0
      MaxSpawnAdd: 0
Worlds:
  worldname:
    somespawntype: true
    Mob:
      Mobname:
        SpawnEveryX: 1
        MinSpawnAdd: 0
        MaxSpawnAdd: 0
        Biome: DESERT
        Block: 12
  another_world_name:
    somespawntype: true
    anotherspawntype: true
    Mob:
      mobname:
        SpawnEveryX: 2
        MinSpawnAdd: 3
        MaxSpawnAdd: 6
        Health: 4

The possible spawntypes are listed here: List of spawntypes. To enable manipulating it, set it to true, else set it to false (default).

"Mobname" and "OtherMob" are for example Creeper or Pig. (List of mobs)

SpawnEveryX says, how many mobspawns are canceled. If you set it to 1, every mob minecraft intends to spawn will spawn. If you set it to 2, every second mob will spawn, if you set it to 3, every third will etc. If it's set to 0, the mobs will never spawn. MinSpawnAdd and MaxSpawnAdd set the boundaries of how many mobs are added to a mob minecraft spawns. If you set both to 2 for zombies, everytime minecraft spawns a zombie, three will appear instead of one. Beware that when the maximum limit is too high, you might experience lags! Health alters the health of the mob when spawning. The value has to be between 0 and 20! If you want the default value, set it to -1 (default). MinLightLevel and MaxLightLevel set the boundaries of the light level required for the mob to spawn. The value has to be between 0 and 15! This only limits it, it does not expand the boundaries set by minecraft!! Biome limits the spawn area to a certain biome. The biome name has to be caps! You can find a list of all possible biomes: List of biomes Block limits the spawn area to a certain block. This can either be the block number or name of the block.

"worldname" and "another_world_name" are names of worlds with their specific configuration.

"Default" is the default world config which is used when no specific world config could be found.

If you don't want a setting, you can just remove it from the config. The following config is totally okay, too:

Worlds:
  equestria:
    NaturalSpawn: true
    Creeper:
      Health: 2
    Zombie:
      MinSpawnAdd: 1
      MaxSpawnAdd: 1