How to edit mobspawn rates?

Hello all. All I ask is to know how to simply edit the rates at which mobs spawn to make my survival/adv game (playing with a friend) a bit harder. Are there any plugins that could maybe do this as well?

Again, I cannot stress this enough, it would be great if it was simple and easy to use!

Thanks in advance!


Edit: also, as a side note bukkit is giving me an error loading my .yml config for a plugin that deals with mob spawn rates. It claims mapping values are not allowed here. If I pasted the config.yml text, would you guys be able to help me out?