Inspired by the thread Spotting a Familiar Actor in a Surprising Place, and my insipid posting of finding Star Trek actors.
Mr. Atoz in 77 Sunset Strip, Dial S for Spenser.
Dr. McCoy in *Have Gun-Will Travel, The Treasure.
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James Doohan, George Takei, and Ethan Phillips on Homeboys in Outer Space.
Shatner in at least 2 twilight zone episodes. One was the famous episode where he saw a gremlin on the wing.
DeForest Kelly in The Lone Ranger (twice) and Bonanza (twice). He even played a doctor in one episode of each series.
Leonard Nimoy as an Indian in Daniel Boone and he also appeared in Bonanza.
James Doohan in Hazel. He was easy to spot. He played a Scotsman. 
Walter Koenig had a cameo as a police sergeant in an episode of Columbo in which William Shatner played the murderer.
Not a TV show, but Nichelle Nichols played a madame in the movie Truck Turner, starring Isaac Hayes.
Before he was famous, William Shatner was in The Twilight Zone twice and in The Naked City.
Nimoy in Zombies Of The Stratosphere.
Rene Auberjonois, or Odo from DS9, played Frasier’s psychology mentor from school in a few episodes of Frasier.
Scotty was in a Bewitched episode. He played a father of a boy warlock who was staying with Sam and Darrin for the weekend.
Arlene Martel, who played Spock’s betrothed in “Amok Time”, also appeared in Mission: Impossible after Leonard Nimoy joined the cast. Apparently, she also appeared with Leonard Nimoy in an episode of The Rebel, but I’ve never seen the latter.
Jonathan Frakes on North and South (soon to be edited and re-released as North.)
Brent Spiner on Night Court.
Rene Auberjonois on Wonder Woman (and with a costarring role on Benson.)
Nimoy was on an episode of Perry Mason…Shatner was in the movie Judgement at Nuremburg
He was also the priest in the original movie version of MAS*H
Leonard Nimoy in The Brain Eaters.
George Takei in The Green Berets.
Yeah, I saw that one when I was a young Trekkie in the 1970s.
Recently I saw a Columbo episode in which the guy who played “Charlie X” was a suspect, and the other day I saw an episode of E.R. which featured the woman who played “Leeta” on DS 9 (she was a “dabo girl”).
Oh, and I haven’t seen National Lampoon’s Senior Trip, but I find this photo of Ezri Dax and Twilight Sparkle amusing. (Rule 34?)
Kate Mulgrew (Capt.Janeway)played in the soap opera ‘Ryan’s Hope’
Takei also appeared in an episode of Twilight Zone.
Shatner screamed like a little girl in, “The Devil’s Rain.” John Travolta was somewhere in there, too.
Nana Visitor was a platinum blond in a Matlock episode.
Of course, Rene Auberjonois was Father Mulcahy in, “MASH.”
Frakes was in the pilot episode of, “The Librarian.” (Directed it, too.)
Cheryl (Gates) McFadden was in, “The Muppets Take Manhattan.”
Charlie (Robert Walker, Jr) X was in a movie called. “Killers Three,” with Dick Clark, who also wrote it.
Sadly, I know that her name is Chase Masterson without having to look it up.
Colm Meaney was Miles O’Brien on STNG and a major character on Hell on Wheels (Thomas Durrant).
John De Lance was Q on STNG, and appeared as the Donald Margolis, who was Jesse’s Girlfriend’s dad on Breaking Bad. I’m pretty sure he was on an episode of Quantum Leap, too, playing the ass-hole millionaire who get’s the Scrooge treatment to turn his life around.
Matt McCoy was Troi’s love interest, Devinoni Ral, and then showed up in the Serenity Now episode of Seinfeld as Lloyd Braun.
Jimmy Doohan thought he killed Richard Harris in, “Man in the Wilderness.”