So about a year ago I posted
this thread about a program called Sky Panorama.
Well today I found a much better space skybox generator called Spacescape. You can view the thread for it
here. There's screenshots and an introduction video. It supports nebulae, stars, and billboards (planets would be an example of a billboard). The best part is that it's free and it can export both jpg and png textures up to 4096x4096.
I made this sky the first time I played around with the program in about 5 minutes.