I like finding actors in performances before they did Star Trek. The most well-known occurance is probably Shatner and Nimoy appearing in the same episode of The Man from U.N.C.L.E., a full year before Kirk and Spock. Shatner, Nimoy, and James Doohan all appear in episodes of the original Outer Limits, as well as several future guest stars. Nimoy shows up in an early episode of Bonanza ("The Ape") as a slimey cardshark, available for free download at the Internet Archive.
I remember seeing DeForest Kelley and Walter Koenig in separate episodes of Perry Mason
. I was struck by what a good actor Koenig was when he wasn't saddled with a phony accent, something he also proved on Babylon 5
Then there was the 1962 Bonanza
episode "The Decision," guest-starring Kelley as a doctor who'd been falsely convicted of murder -- and John Hoyt as the judge who sentenced him to death!