JasonF, are you wanting 3D or 2D clouds? Also, what depth of realism are you looking for? I've done a few 2D could renders, but they produce a very commic style cloud and not a realistic one. As for 3d ones, I couldn't find much that you probably haven't already found, but a page of links I found useful was http://www.vterrain.org/Atmosphere/Clouds/

If you want 2D a very easy way is to create about 10 different patterns in any paint program. Use them as Alpha Masks and combine them at random to build out a cloud, then texture/light it as you see fit. I found that using a perlin generator worked best as it created more symmetry then a standard random function. Random seemed to produce things that just didn't look like a cloud .

Of course, generating the cloud is only 1/2 the battle, you still have to make it react properly to the environment. I never took it this far, and instead had my clouds simply float over everything on the screen.