Nimbus is an experimental cloud generator system. Its far from complete and states that its still under development. The interesting thing is the python script for Blender that could make creating cloud shapes with particles more easily.
Quote from website on the script:
Artists broadly define a cloud's shape using overlapping boxes. The Cloud Creator will randomly generate particles within that volume when the "Create Cloud" button is pressed. The randomly generated particles are created within the parameters set in the cluster of buttons at the top of the script: the number of particles, the particle size range, and the particle rotation range.
Find it here.