I might be alittle  biased because I always love a plugin that makes it easy for me to implement custom sounds effects, especially npc voices - but this deserves 5 stars. BadKop's plugin does exactly that. Not only does it allow you to add your own voice clips and sounds, there's provided excellent recordings and its very easy to set up. BadKop has a quality sense of humor and plugin design skills.

This has made my day and allowed me to make my server a much more creative and funny experience. 
Hope to see more like this.