I've been looking for programs to do it for a while. Terragen is great, and it has a lot of options. For the people who don't want to spend an our tweaking options, I found this one today and it looks promising:
http://www.blitzbasic.com/Community/posts.php?topic=31940I don't have the time to experiment with it at the moment, but you're welcome to take a look at it yourself. If you find out anything, be sure to share. It's very simple to use - Press Z or A if you want a sky or a place in space. Then press F1 to render. Then press S to save. It saves as a huge BMP files - 3072x512 and around 4.5MB average. It looks like it would be pretty simple to divide it into 6 pieces (512 divides into 3072 exactly 6 times and there's six sides on a cube, what a coincidence), convert it to the right file format (jpg) and make a pretty skybox. Try it and post some screenshots from ingame.
Space renderings generate a lot faster than a sky rendering. Overcast skies are pretty ugly (and you can tell when you're getting them because the program slows down a lot on the lighting clouds stage), but some of the other skies are quite nice. It really surprised me when I got my first space map with a planet, I didn't realize they were in there. It was realistically lit and had a cool atmosphere glow effect. I was even more surprised when a nice red planet appeared in the day sky.
One drawback of the program is that there are absolutely no options. You have to have a blue sky. You can't change the number of stars. What you get is randomly generated. Skies also have a hole in the bottom because of the way the sky is generated. Most of the time you won't be able to see it anyway because there is fog and terrain in the way. You might want to stick to terragen or another program if you want a bright pink sky (although you can always run your sky through a few filters on photoshop if you want) or if you want people to be able to see the bottom of your sky (the only map I can think of that ever needed this was Celestial Dreams).
I'll post some pictures later.
