You really don't want to get me started on this... Face-spotting is a minor hobby of mine
Pat Tallman - Lyta Alexander in Babylon 5 - also a mercenary in either ST:TNG's Starship Mine or Gambit (I think).
Bill Mumy - Leneir (B5), Starfleet soldier in Siege of AR558 (DS9), Will Robinson (Lost in Space).
Sarah Douglass - Deathwalker (B5), Female Romulan Commander (TNG).
Nicole DeBoer - Ezri (DS9), also in Deepwater Black and The Cube.
Jerry Doyle - Garibaldi (B5), Captain Simian (Captain Simian & the Space Monkeys).
Michael Dorn - (do I have to say?
), the bad guy (Captain Simian).
Andreas Katsulas - Tomalak (TNG), G'Kar (B5).
Mark Alaimo - Dukat (DS9), assassin (Last Starfighter).
I could go on forever...
Of course David Lynch's Dune is worth checking out for this kind of thing:
Kyle McLachlan - Paul -> Agent Cooper (Twin Peaks).
Patrick Stewart - Gurney Halleck -> it's on the tip of my tongue...
Brad Dourif - Piter de Vries -> assorted psychos in Voyager, B5 and the X-files.
Dean Stockwell - Dr Yueh -> Al (Quantum Leap).
And about half the cast of Twin Peaks...
Jon
"There are worlds out there where the sky is burning, where the sea is asleep and the rivers dream; people made of smoke and cities made of song.
Somewhere there's danger, somewhere there's injustice, and somewhere else the tea is getting cold. Come on, Ace, we've got work to do."
THE DOCTOR, "Survival" (Doctor Who)