Adam Baldwin has been on Firefly, Angel, and Stargate:SG-1
Wayne Alexander has played about 15 different non-regular roles on Babylon 5 (I think Lorien was the only recurring one), and also has credits for X-Files, Sabrina, the Teenaged Witch (if fantasy counts), Hypernauts (which I’d never heard of, but I presume is SF), Otherworld (likewise), and The Twilight Zone. He was also in Spaced Invaders, if we count movies.
In addition to the Twilight Zone (and Outer Limits, as Dr Rieux noted), there’s another connection, somewhat “fringey” as far as strict SF goes:
For the first few seasons of Mission: Impossible, real-life husband/wife team Martin Landau and Barbara Bain portrayed the “gadget man” and what I’d have to call the token woman on the IMF team. When they left the series, after ST:TOS ended, Leonard Nimoy took the role of Paris, the replacement “gadget man.” During this period, Shatner was a regular guest star, portraying a number of villains whom the team was sent to combat. (Probably some of his best acting, too – much more complex roles, requiring establishment of the character in a much tighter time frame, than Kirk.) There are rumours of a parody script of an episode pitting the IMF-with-Paris against a mysterious Shatner villain, with the denouement showing Paris and the villain whipping out cell-phone-like devices and ordering “beam me up.” The familiar transporter effect is shown, they vanish, and Peter Graves et al. are left with quizzical expressions on their faces. Freeze frame.
As did George Takei, who also had a couple of appearnaces in Heroes (if I remember the ads correctly.)
Before there were television shows, there were Saturday morning serials at the local movie theater that pretty much served the same practical application. Leonard Nemoy played Nerab (sp) in one of the great titles in science fiction history: Zombies of the Stratosphere. I understand later Nemoy went on to further fame in being an alien in a weird uniform, but the specifics slip my mind.
Shatner - TekWar / what was that other one called?
John Simm - Life On Mars / Doctor Who (later this year in the series finale)
Tom Baker - Doctor Who / Randal & Hopkirk (deceased)
Latvian-born actor Elya Baskin plays a Soviet cosmonaut in 2010: The Year We Make Contact, and Peter Parker’s kindly landlord in the Spider-Man movies: http://www.colorofthecross.com/images/c_elya_final_3_.jpg
John Hurt played Kane in Alien, Winston Smith in 1984, Prof. Bruttenholm in Hellboy, and British fascist leader Adam Sutler in V for Vendetta.
Shatner and Nimoy appeared in the same episode of The Man from U.N.C.L.E.!
I saw the Stargate:Atlantis episode with Brandy Ledford. I knew she seemed familiar - so I did an IMDB lookup. She was Doyle on Andromeda (she was also on Stagate: SG! as a different charter as teh one she played on Atlantis)
Also apeard on Smallville, The Invisible Man, and Whistler
Brian