CoolClouds is a fully customizable clouds plugin that allows server owners to create there own unique clouds, this plugin is like none other that I have seen and is extremely user friendly. CoolClouds has one command, /cloud. This command will open a clouds GUI allowing players to select the could they would like to use. Players can also type /cloud <cloudName> and choose a cloud.
CoolClouds has an easy to use configuration file for adding custom clouds.
Here is a list of usable effect types