Well, I'm a GIMP fan, and I made quite a few wallpapers for myself using GIMP, so I'll post them here for anyone who's interested.
These are all under the Creative Commons BY license.
War of the Depths:
More Blue (imaginative, isn't it?):
Swirling of Hope:
Ebb and Flow of Life:
Simplistic Design (tutorial result):
Ghosts in the Machine:
Turbulence of the Soul: