Actor Allusion: This show was the subject of one for John Ritter in Stay Tuned, a movie about people getting trapped in a series of TV shows. He briefly ends up on the set of Three's Company, screams at the fourth wall, and escapes to another channel.
David Ruprecht, later to become famous as host of the Lifetime/PAX game show Supermarket Sweep, appeared as Phillip Dawson, the man whom Janet married in the final episodes. Incidentally, Sweep's creator and executive producer Al Howard previously worked for ABC himself as an account executive and advertising copywriter, and in fact, Sweep originally first ran on ABC's daytime schedule from 1965 to 1967.
Peter Mark Richman, an eminently recognizable face on TV, plays Chrissy's father Reverend Snow in several episodes.
Anne Schedeen (aka Kate Tanner) appears in five episodes over the course of the series: she plays Jack's girlfriend Linda in "Will the Real Jack Tripper...", "Double Date" and as a replacement for Janet in "Stanley's Hotline" (the only episode in the series in which Joyce DeWitt doesn't appear). Also, Anne plays another of Jack's girlfriends, Lisa Page, in "Honest Jack Tripper" and Louise Prescott in "Jack Gets His Own Restaurant."
Jeffrey Tambor plays three different characters in as many episodes: Winston Cromwell III in "Father of the Bride," Dr. Miller in "Two Flew Over the Cuckoo's Nest" and Dr. Greene in "Jack Goes to the Dentist" (he also played the Ropers' uptight neighbor Jeffrey P. Brookes in their spin-off series).
Natalie Schafer (aka "Lovey" Howell) plays a customer in Janet's flower shop in the 2nd-season episode "Jack in the Flower Shop."
Loni Anderson plays Jack's old flame Susan Walters in the 2nd-season finale "Coffee, Tea, or Jack."
In the 3rd-season episode "The Crush": Laurie Hendler (aka Julie Kanisky in the TV series Gimme a Break!) plays Laurie and Steve Shaw (Eric Fairgate in Knots Landing) plays Albert.
In the 3rd-season episode "Jack Moves Out": Cynthia Harris (aka Sylvia Buchman in Mad About You) plays the horny Mrs. Layton, and John Larroquette (Night Court, Boston Legal) plays the cop that comes to their apartment (Classic line: After finding out Jack lives there with two girls, he says "Never mind why you broke in. Why did you break out?!")
'80s TV buffs may recognize Barrie Youngfellow, who plays Jack's girlfriend Debbie (who's living with two guys!) in the 3rd-season finale "Triangle Troubles"; she played Jan Hoffmeyer in the series It's a Living.
Extremely prolific character actor Phil Leeds (best known, perhaps, for Ghost and Rosemarys Baby) plays Lyle Wormold in the 4th-season episode "The Life Saver."
'80s TV buffs may also recognize Philip Charles MacKenzie, who plays Roger in the 4th-season episode "And Baby Makes Two." He was also Donald Maltby in the Showtime comedy series Brothers from 1984-1989; in the '90s and '00s he has been a very busy director of TV shows (According to Jim, Frasier, Roseanne).
Teresa Ganzel, who played "Art Fern's Tea Time Movie Lady" on Johhny Carson's Tonight Show, and is a prolific voice actress on animated TV shows and Pixar films as well, plays the oft-spoken-of "Greedy Gretchen" in the 6th-season episode "Lies My Roommate Told Me" (she reprised that role in an episode of the Sequel Series, Three's a Crowd.)
Alan McRae, who played Samuel Douglas Sr. in the 3 Ninjas franchise, plays the robber in the 6th-season episode "Eyewitness Blues."
Karen Austin (Night Court, Summer Rental) plays Jack's controlling fiancee Denise in the 6th-season episode "Jack's 10."
The late Edward Andrews (Grandpa Howard in Sixteen Candles) played Jack's grandpa (who thinks Jack is a doctor) in Season Six's "Doctor in the House."
Barry Williams (aka Greg Brady) plays the object of Janet's desire, David Winthrop, in the 6th-season episode "Up in the Air."
Lucille Ball plays herself as the host of the Clip Show that wraps up Season 6.
The late Stanley Kamel (Monk, Melrose Place) played the "Clean-Cut Man" in the 7th-season episode "Opening Night."
Earl Boen (aka Dr. Silberman) plays Reverend Gilmore in Season Seven's "The Brunch."
Beau Starr (SheriffBen Meeker; Lt. Harding Welsh in Due South) plays a police officer in the 7th-season episode "Breaking Up Is Hard to Do."
As with the J. Pat O'Malley example above, another extremely prolific character actor of '50s, '60s', 70s, '80s and '90s TV, Parley Baer, played Bert Landers in Season Eight's "Grandma Jack."
Old Shame: "Shame" might be stretching it, but Priscilla Barnes says she was miserable the entire time. She's called it the three worst years of her life, and tried to walk, but couldn't get out of her contract.