Terragen sky cubes are nice as well. You can render cool mountains. I used that for a few of my images. But the clouds are very simple. They have some kind of a mosaic effect applied and look very flat.

The clouds from Lightwaves sky tracer have more volume, shadows and better lighting.

Some people are still waiting for night sky cubes. But as I mentioned earlier I am not really happy with it. When you use details like stars in a reasonable resolution like 512 and you play your game with 1024 or more then the stars are simply blurry. Tey have to be resized from the engine to the double of the origninal size and even more in a higher resolution. You can see that in almost every game that uses night sky cubes (even Doom3, Quake4 and so on).

But this problem is not an issue for day sky cubes. There are not so much small details like stars, galaxies or mountains.

But when you use a 6x1024x1024 sky cube then it will take up to 20 MB of memory in your graphic card. That is simply the reason why all those AAA games use such blurry and small sky cubes.

But nevertheless I will present some night sky cubes soon. But I have to work on many projects at the moment.