If you have a DVD player, odds are you already know about Netflix's excellent movies-by-mail service. Netflix is morphing into a similarly excellent streaming-video service, but that's no reason to forget the mailman's name. There is an array of niche "flix" companies that specialize in delivering media to your door. SmartFlix specializes in how-to tutorials; Fight-Flix, in martial-arts videos; BookSwim and Paperspine, in books; and, dwarfing them all, GameFly, the Netflix of video games.