If we're going to dip into the voice acting pool, then both Michael Dorn and Rene Auberjonois have been to the Mojave Wasteland, as Marcus the Mutant and Mr. House, respectively. Not sure if any other Trek actors have lent their voices to the Fallout series, though.
not sure if we are counting fan productions as well but george takei and barbara luna both were in star trek phase 2 new voyages episodes . george was in time enough while barbara was in several that i know of. oh and denise crosby was in a episode as well.
If we're dipping into the computer games pool, I'm pretty sure Michael Dorn voiced a Might & Magic VII goblin who's rather insulted whenever a shopkeep calls the party cheapskates . . .
I'm sticking with TOS, which is how this started over at TBBS. Another UNCLE episode, right after the william Marshall one, featured Alfred Ryder (Man Trap). And I saw a Mannix Episodes with Barbara Babcock, Lee Meriwether and that guy from Bread and Circusses, and another Mannix with Sabrina Scharf (Miramaneeeee!) and the bereaved husband from Wolf in the Fold.
I guess this kinda counts: [yt=Nana Visitor on PBS Newshour talking about Hurricane Sandy]o_96CNmloDo[/yt] Still a hottie.
Mark Lenerd for Nova. He was also a regular in the short lived series, Here Come The Brides, co starring with Michael? Brown from Who mourns for Lazarus
W00t!! Marianna Hill (Dr. Helen Noel, Dagger of the Mind) on a 3rd season Mannix ep titled A Sleep in the Deep:
Here's one I didn't expect - a crazy sexy lady with a gun in Night Court, season 6, "Educating Rhoda"
A few nights ago Sylvia from 'Catspaw' and Garak were in the same "Kojak", sharing some scenes together & interacting.
Andrew Robinson was in the first Hellraiser, whilst Terry Farrell was in one of the later ones, 6 I think. John DeLancie pops up in plenty of things - he's the bad guy in one of the Get Smart spin-off movies. Yet to see him stretched beyond Sneering Guy, but since he sneers so well Warehouse 13 mines the Sci-Fi/Fantasy actors quite well, with Spiner, Mulgrew, Ryan, Auberjonois all having had sizeable roles, with a fair few Buffy and Firefly alumni making appearances.
Anyone mentioned Michael Ansara and Leonard Nimoy appearing as brothers who are lawmen in a violent town in an episode of "The Virginian"? The episode is clearly meant to be a homage to the legend of the Gunfight At the OK Corral with Ansara and Nimoy's characters as the Earp brothers facing off against the cowboys.
He had a good, serious role in Breaking Bad Season 2, playing the father of Jesse's girlfriend. When Walt (Bryan Cranston) allows something...bad...to happen to her, it sets in motion events involving DeLancie's character which lead to a tragic conclusion.
IIRC, Michael Forrest (Apollo, "Who Mourns for Adonis") and Michael Ansara (Kang) were both in an early episode of the Rifleman. Where an Indian chief (Forrest) kidnaps Lucas McCain's son Mark to replace his son that has died.
And we watched a 1962 episode of Naked City last night with Roger C. Carmel as a businessman with a horrible secret in his past. He didn't have much hair then, either.