David Warner - He was in TNG, B5, and two Trek films as different characters. Not to mention Jack the Ripper in Time After Time.
Michael Ansara - In TOS's "Day of the Dove," in Buck Rogers as Kane, and in B5 as Elric the Technomage.
Bill Paxton - Wasn't he in Near Dark? Plus Aliens, the Abyss, Apollo 13 (yeah, not exactly SF, I know,) Weird Science, and some other stuff.
Brian Thompson - He was the killer in Cobra. Also the Brujah Leader in Kindred: The Embraced, and in Buffy twice, as Luke, the Master's lieutenant, and as that demon guy (the Judge) that Buffy took out with a rocket launcher.
Kyle MacLachlan - Dune, Blue Velvet, the Hidden, and Twin Peaks.
Rutger Hauer – In so many genre films you can’t even count them. Starting with Blade Runner and Ladyhawke.
Freddie Jones - Is he dead? He played Thufir Hawat in Dune and was in Firestarter and Krulll.
Richard Jordan - Also in Dune, and in Logan's Run. He's definitely dead.
Max Von Sydow - Ming the Merciless. 'nuff said. EDIT: Actually, he's played Jesus, the Devil and Ming the Merciless. Samuel L. Jackson wishes he were that cool.