Pioneer of a opera/synth pop mix (and only person to combine opera and modern music sucessfully in my opinion). I only recently discovered him.
He reminds me of Joy Division because of the space-influence (and thus Bowie of course as well). He had a highly stylized "automaton" fashion that made him seem like he came from another world...
His stage performances have a clear influence from the Space Age as well as the art school Bauhaus (not the band, but the school from which the band rendered it's name).
Many of his songs are "dark", and definatly remind me of some of the ballads ala Cinema Strange.
Klaus and Bowie performing on SNL
Excellent website that features Nomi songs on many of it's page:
http://perso.orange.fr/klaus.nomi/home_ve.html