Kraftwerk: Duh. Especially since they use synthetic vocals extensively, any vocals performed by a person try to sound like they're synthetic, and many of their live shows actually do involve robots controlled through midi software to play backup; the four robots have molded replicas of the real men's heads.
Skrillex had a female alien as his alter ego which he controlled via motion technology on a few tours.
Starset perform wearing illuminated face masks and fake oxygen tanks. In their kayfabe, they claim that their lyrics were beamed to them by aliens.
Planetoid's kayfabe is superficially similar to that of Gwar: They're aliens from the future who found themselves stranded on earth, and who now intend to conquer the planet with The Power of Rock. They actually fit into both the "spaceman" and "robot" categories, since drummer Admiral Time is supposed to be a cybernetic organism as well.
Future Folk are General Trius and The Mighty Kevin, who were originally sent to earth from the planet Hondo to kill off all human life and start a new settlement there. Fortunately, they soon became fascinated with the human invention known as music, and decided to spare humanity and focus on performing original folk songs. Their back-story was made into the movie The History of Future Folk: In the film, audiences think they're a comedy duo with spaceman alter egos, when in fact they actually are from outer space.