Alan Cumming was in "The Flintstones in Viva Rock Vegas (2000)" playing The Great Gazoo, a space alien. He co-starred with Kristen Johnston as Wilma Slaghoople. In this episode, Johnston is the alien and Cumming is the human.
John Cleese of Monty Python guest-starred 4 times in 3rd Rock. Sally's response to Don that Dick & Harry are witches if they don't float when she floods the hole with water echoes a classic scene from Monty Python & the Holy Grail when the knights are trying to determine whether an accused woman is really a witch by whether she floats.