What is the best way to make sky?

What is the best way to make sky/clouds? Either in blender or another program… I tried Terragen, but I was wondering if anything else was good.

This tutorial might be of some use:

http://www.cogfilms.com/CQTs-5.html

One of the google SoC projects for blender is also a sky generator, but this won’t be usable for some more time.

http://mediawiki.blender.org/index.php/User:Sionix/SummerOfCode2006

Looks like a good tutorial. Thanx a lot.