Aren't many choices!? In addition to Ruger, Remington makes both SxS and O/U doubles, Mossberg makes an O/U, Kimber makes BEAUTIFUL (and quite expensive) doubles, Marlin produces doubles under the L.C. Smith brand, BSA makes doubles, so does Savage....
I'm sure there are quite a few others, but those are the major manufacturers I could come up with offhand.
Jump on google, see what else you can stir up.