Hey! You got your TREK in my ZONE!
Here is a fun tally of STAR TREK actors who were also in THE TWILIGHT ZONE, courtesy of Luvinthe60s:
Nightmare at 20,000 Feet (William Shatner)
A Quality of Mercy (Leonard Nimoy)
Valley of the Shadow (James Doohan)
The Encounter (George Takei)
Twenty-Two (Arlene Martel)
Of Late I Think of Cliffordville (Julie Newmar)
The Long Morrow (Robert Lansing, Mariette Hartley)
People are Alike All Over (Vic Perrin, Susan Oliver)
Dead Man's Shoes (Warren Stevens, Joan Marshall)
Probe 7 - Over and Out (Antoinette Bower)
The Lateness of the Hour (John Hoyt)
The Last Rites of Jeff Myrtlebank (Sherry Jackson)
Five Characters in Search of an Exit (William Windom)
The Passersby (James Gregory, Joanne Linville)
Vic Perrin did a lot voiceover work for TREK as well.
I have a feeling this is not exhaustive. Even though I haven't seen every episode of TZ, I know that Shatner was in another episode, Nick of Time. Someone could do the same for THE OUTER LIMITS -- and list of actors would be similar!
And, while we're on the 1960s crossover topic -- here are some TREK actors who were on WILD WILD WEST, thanks to Ptolemy1961: :
And while we're on the subject, here is the WILD WILD WEST theme mashed up with the UFO theme. And it all fits real nice, too!