I just found a (for me) new app that is 1) free 2) iPad and iPhone/iPod compatible 3) perfect for ambient and thought I should share it!

The name is Orbita, and it's based making droning sounds by making "planets" spin around the center, the closer the planet the louder the volume. I realize this explanation sucks, but since it's free just try it for yourselves

I think the actual sound is vibrations from various planets converted to audio, which is also cool