360x90 degree spherical Skybox textures, UV map them to a hemisphere for best result.

- (old)  sky_*.jpg also d* m*
Free high res skymaps (Massive 07 update!) 
by: 0ptikz
http://blenderartists.org/forum/showthread.php?24038-Free-high-res-skymaps-%28Massive-07-update

- (new)  gen_*.jpg, sky-*.jpg
New High resolution sky pack for Blender
by: 0ptikz
http://blenderartists.org/forum/showthread.php?224065-New-High-resolution-sky-pack-for-Blender

Released under an 'I don't give an damn' license.
Meaning you can use them for anything you want, including commercial work.
The only thing I ask is that you don't sell them. That's it.