Guess I must be in an 'experimental mood' at the moment!
This particular one came from directions on the 'Try It Tuesdays' site (which isn't in operation at the moment) and is called the Easy Rusted Background...original technique was by Judy Scott.
Basically it involves a fairly thick coat of gesso, little pieces of aluminum foil, salt or sand, and two or three colours of paint. I used four colours of acrylic folk art paint for my sample (again using up some of my stash...some of my paints are getting pretty yucky!!). I like the end result, but would try some different colours next time. My colours are pretty old and certainly aren't 'in vogue' right now.