A while back (was it really over a year ago) I started collecting some real nebula photos to start adding some more variety to my backgrounds. The technique I use is to take a real nebula photo and process the photo, sometimes completely by hand, to extract all the clutter/stars so that I can later re-composite them back into my scenes. I’ve got a technique that works pretty well now for many images. Specifically, so I don’t have to hand edit as much with the clone tool. I do all my bitmap work in GIMP, which is a wonderful open source graphics package.
OK, so the last one isn’t a nebula, but same technique.