These skyboxes are made exclusively with procedural Blender materials, Gimp filters and Inkscape, i.e. they are completely algorithmic.
The packets include all the source files (.blend, .xcf, ...), the skybox images (6x 1024x1024), an editorimage and a script.
Planets

http://rapidshare.com/files/296107295/newsky_planets.pk3 (12.6 MB)
EDIT: I also uploaded the terrestrial planet material to the Blender Open Material Repository.
Asteroids

http://rapidshare.com/files/296107293/newsky_asteroids.pk3 (26 MB)
Nebulae

http://rapidshare.com/files/296107294/newsky_nebulae.pk3 (13.6 MB)
(I know that some of my methods to get things done with blender are questionable, I'm still learning
