Here's some other TOS regulars before TOS:
George Takei from "The Twilight Zone". This episode has not been seen on TV since it first aired, although it has been released on video and DVD.
Here's a couple from "The Fugitive". The first with Jimmy Doohan. The second with DeForest Kelley.
And, of course, we can't forget that Shatner was on "The Fugitive", too.
Here's a couple of more with Kelley, from separate episodes of "Route 66" . . .
. . . which, incidentally, was ANOTHER series that the ever-present Shatner was on.
Three more youtube clips - these are from separate episodes of the 1960's high school drama "Mr. Novak", a series with a lot of Star Trek connections, and each features Walter Koenig:
Koenig's full-length Alfred Hitchcock episode, "Memo From Purgatory", can also be seen at hulu.com (as can Shatner's).