This is the bottom half list of guest actors in Star Trek roles other than what they were famous for. It includes everyone appearing in the different series, in no particular order.
Before he was Zephram Cochrane in Star Trek: First Contact,
James Cromwell guest starred in The Next Generation and Deep Space Nine. In the episode "Birthright I & II", he was the most unrecognizable as a Yridian.
Gwynyth Walsh's Lursa made it to the big screen in Star Trek: Generations. After her long stint as a Duras Sister, she was a member of a telepathic race in Voyager's "Random Thoughts". I can't help but notice that parts of her uniform appeared in previous episodes as well.
You all knew him as Kang. After playing the Klingon,
Michael Ansara was Lwaxana's husband. Their marriage was annulled, when Odo took Lwaxana as wife so she may keep their unborn son in "The Muse".
Scott MacDonald actually started acting in television in Star Trek: The Next Generation. Before he was Tosk in Deep Space Nine and the Xindi Reptilian Dolim in Enterprise, his first role was a Romulan in "Face of the Enemy". To see what he really looks like, watch Voyager's "Caretaker". He was on the bridge.
I have more, but these are the more notable of characters.
