His nearly 150 credits on IMDB.com certainly show solid inroads into Horror -- the genre he's largely most recognized from -- but there's some meaty SciFi and Fantasy work stuck in there as well if you're looking closely. Maybe I'm just an "old coot" because I certainly remember him from appearances aboard TV's camp classic Batman, the original (and still the best) Star Trek, and the Saturday morning classic Jason Of Star Command. Still, I'd honestly completely forgotten THX 1138 (1971) and his visits to Buck Rogers In The 25th Century. Sid had so many great opportunities to chew scenery, and chew with the best of them he did.
Alas, the man has passed on now, but his work remains, and it will no doubt continue to inspire. Even when cast as the heavy (typecastig was something that the actor learned to resent about the entertainment business), he turned in solid work as somehow worthy of your fear AND admiration.
May he rest in peace ...