Stanley Adams, Cyrano Jones in The Trouble with Tribbles, in Peter Gunn, Sing a Song of Murder. He was a jazz composer.
The actor, Janos Prohaska, was also the Horta in Devil in the Dark and Yarnek the rock creature in The Savage Curtain.
Barbara “Mea 3/Philana” Babcock as “Grace Gardner” in Hill Street Blues and “Lana” in the Cheers! episode “Now Pitching, Sam Malone.”
A rerun of “Barnaby Jones” was on last night with *Bill Zuckert (TOS: “Spectre of the Gun”) and Meg Wyllie (“The Keeper” from “The Cage/Menagerie”)
And as a bonus in the same ep: Mark Goddard from “Lost in Space.”
(And, unrelated to the thread, a brief appearance of a young Nick Nolte.)
*Zuckert was one of those actors who was pretty much everywhere, although you seldom remember his name. Somewhat similar to Ken Lynch.
Valerie “Nevala” Wildman (VOY, “Message in a Bottle”) in the St Elsewhere episode “Ties That Bind.”
Here’s an obscure one, and a rather surprising spot:
Sid Haig.. He played First Lawgiver in the TOS episode “Return of the Archons”. He’s mostly known for roles in horror movies.
But I didn’t see him in a horror movie - instead, I saw him on a TV cooking competition show, as a judge, when people were competing to design Halloween-themed displays (Food Network / Halloween Wars). I looked him up afterward; turned out, he had died just a month or so earlier; I don’t recall whether the show was a repeat of an earlier year or had simply been filmed just before his death.
Yet another ST–Batman crossover: He played “The Royal Apothecary” in a second-season “King Tut” outing.*
*No pun intended.
Madlyn Rhue played Marla McGivers, the historian who aided and went into banishment with Kahn Noonian Singh, was in the Route 66 episode, Every Father’s Daughter. I recognized the actress, but had to consult IMDB and Memory Alpha to find her name.
Here’s one for you: A very young* Clint “Balok” Howard in this series, the history of which is interesting:
https://en.wikipedia.org/wiki/The_Baileys_of_BalboaHow could anybody not like Gilligan’s Island? 
*Two years before he appeared on Star Trek.
Turns out Michael J Pollard (“Jahn” in “Miri”) had a minor role in TBoB as well.
Quite a lot of ST actors played in bad horror movies before AND after their stints on Star Trek.
https://www.syfy.com/syfywire/13-embarrassing-horror-movies-star-trek-actors
Chris Pine did a credible performance in the movie version of Into the Woods - even sang somewhat competently.
This one is The Mother of Them All. Why, why, why was it left off the list?!? :dubious: 
Mickey Moore (“Kloog” in “The Gamesters of Triskelion”) as a loan-shark enforcer in the Hill Street Blues episode “Fruits of the Poisonous Tree.”
Jane “Amanda” Wyatt (Spock’s mother) as “Mrs Katherine Auschlander” (Dr Daniel Auschlander’s wife) on St Elsewhere.
He was also in a couple of episodes of The Untouchables. In one, he ran afoul of The Mob and was whacked in the backseat of a roadster.*
*It was painfully obvious that the pistol wasn’t even pointed at him. :rolleyes:
Charlie X, Robert Walker, Jr. in the Route 66 episode, Across Walnuts and Wine. He must have been twelve.
Jonathan “CDR William Riker” Frakes as drug-dealer/ripoff man “Eddie Sims” in the Hill Street Blues episode “Of Mouse and Man.”
Morgan “Captain Ron Tracey” Woodward as “John” (Andy Renko’s father) in the “Zen and the Art of Law Enforcement” episode of Hill Street Blues.
William “Commodore Decker” Windom as “Charlie Halloran” (Joan’s father) in the St Elsewhere episode “In Sickness and in Health.”
Reverse issue. Faith Salie (Wait wait don’t tell me and CBS Sunday Morning) on DS9
Brian