Bukkit

Everything is configured in the config.yml

It has the following layout:

MobspawnConfig:
  spawntype: true
  Mob:
    mobname:
      SpawnEveryX: 1
      MinSpawnAdd: 0
      MaxSpawnAdd: 0

"spawntype" is for example NaturalSpawn. To enable manipulating it, set it to true, else set it to false

"mobname" is for example Creeper.

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 boundries 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.