The Cloud System and its free edition Cloud System Free are available in the Unity Asset Store. It is an artist friendly and time saving tool to create billboard based volumetric clouds in Unity.
The newest versions of the Cloud System require Unity 4. You find an older version here:
Cloud System for Unity 3.x
You find more information and web players on our webpage. For a rough comparison of the paid and the free version, you can check the Cloud System Free page.
If you have any question, suggestion, feedback, criticism or bug report about the Cloud System, feel free to post them here. We are happy to reply to every post!