LemonSpawn VolClouds is a true real-time volumetric cloud ray-tracer for Unity. Lightweight and easy-to-use, the asset basically consists of a single shader and a material.
But don't be fooled by the simplicity - VolClouds simulates realistic-looking clouds that would even stun your mother.
Web player demo
Installation, instructions and information
Youtube video
Note: VolClouds requires a modern graphics card to work, as it is quite GPU intensive. In addition, VolClouds 1.0 only supports rendering of clouds from below the cloud layer - moving through the clouds is currently not possible.