Operas, one would think, provide a plenitude of potential brand names. Cavalleria Rusticana could be a line of ready-made clothing, the Cracker Barrel of the garment business; Mimi Pinson, a chain of home decor outlets; Amneris, exotic lingerie.
But Das Rheingold -- sorry, L'or du Rhin has got to be perfume.
UPDATE. A reader has e-mailed me to let me know that Rheingold Beer was the most popular beer brand in NYC in the 1930s through the 1960s -- they even had Miss Rheingold contests back then. The brand itself was introduced in 1883; I'm not sure the brewers had the opera in mind.
Sure enough, there are all sorts of stuff called Snegurochka, Rusalka and Sadko in Russia -- from candy to sausage to office paper to computers to kindergartens to health centers -- but I doubt Rimsky-Korsakov and Dargomyzhsky are responsible.