Stephen Hawking plays himself in a game of poker with Sir Isaac Newton, Albert Einstein and Data in the "Star Trek: The Next Generation" Episode "All Good Things...". Hawking currently holds the position of Lucasian chair of Mathematics at Cambridge University, a position formerly held by Sir Isaac Newton and held by Data in future scenes of "All Good Things...".
Kelsey Grammer of "Cheers" and "Frasier" played Captain Morgan Bateson, a captain of the Enterprise whose ship was stuck in a time loop, in the "Star Trek: The Next Generation" episode "Cause and Effect".
Kurtwood Smith, best known for "That '70s Show" and "Dead Poets Society" has played several roles in "Star Trek". He played the Federation president in "Star Trek VI: The Undiscovered Country", Thrax, Odo's predecessor in the "Star Trek: Deep Space 9" episode "Things Past" and Annorax in the "Star Trek: Voyager" episode "Year of Hell" and "Year of Hell: Part 2".
Nikki Cox who is best known for her role on the sitcom "Unhappily Ever After" played Sarjenka, a little girl on a doomed planet communicating with Data, in the "Star Trek: The Next Generation" episode "Pen Pals".
It is easy to forget that the Star Trek franchise has been cranking out films and shows for a LONG time. So far there have been six series, "Star Trek", "Star Trek: The Animated Series", "Star Trek: The Next Generation", "Star Trek: Deep Space Nine", "Star Trek:Voyager" and "Star Trek: Enterprise" as well as eleven feature films. Due to the large, ensemble cast and multiple plot lines per episode, this means that at one time or another an absurd number of actors have appeared in minor roles.