Before he starred in The Honeymooners, Jackie Gleason was the original star of TV’s LIfe of Riley.
And I thought Gleason was MUCH better than William Bendix.
It’s spelled Garry Shandling.
It’s spelled Newhart in all shows.
Even if you don’t count AD, Ron Howard was a main cast member in The Andy Griffith Show and Happy Days - both highly successful.
That’s what I was using “Opie Cunningham” as short-hand for. 
:smack:
And now in Life’s Too Short (on HBO), playing himself, as well as doing that voice-over thing for his animated series that I can’t recall the name of.
I’ve never actually seen that show, though I was aware of it when it first came on, and I didn’t even think about it (I pretty much stopped watching sitcoms around the time Seinfeld went off the air). I mentioned “the entire cast of Seinfeld” because of the failures of all of their sitcom ventures following the end of that show (Watching Ellie, Jason Alexander’s Bob Patterson and Listen Up!, and Michael Richards’ The Michael Richards Show), until New Adventures.
I would say those count.
If you’re going to count those, you might as well count “Fat Albert & the Cosby Kids”: within the genre of the Saturday Morning Cartoon, it was a sitcom.
Now I feel bad for Bud.
Speaking of Christina Applegate, it’s a bit too early to say if “Up All Night” will be a success, but the show seems to be doing well so far. And her co-star is Will Arnett - from Arrested Development.
Elinor Donahue was the eldest daughter on*** Father Knows Best ***and Felix’s steady girlfriend on The Odd Couple.
Speaking of The Odd Couple… Al Molinaro played Murray the cop on that show, and later became the new owner of Arnold’s on Happy Days.(Both shows were produced by Garry Marshall, who likes using the same actors over and over).
Ken Berry was one of the stars of F Troop, then went on to be the star of Mayberry, RFD, then went on to Mama’s Family and a few forgettable roles in other sitcoms.
Fred Gwynne was in Car 54 and The Munsters
There is the double connection (Johnny Galecki/Sara Gilbert) between Roseanne and The Big Bang Theory…
How about…
Ken Berry on*** F Troop ***and Mayberry RFD?
Jim Backus on ***Gilligan’s Island ***and I Married Joan?
Eve Arden on ***Our Miss Brooks ***and The Mothers-in-Law
Fred Gwynne on ***The Munsters ***and Car 54, Where Are You?
Pam Dawber on*** Mork & Mindy ***and My Sister Sam.
Meredith Baxter on ***Bridget Loves Bernie ***and Family Ties?
Howard Hesseman in WKRP in Cincinnati and Head of the Class
Valerie Bertinelli in One Day at a Time and Hot in Cleveland
Ted Wass in Soap and Blossom
Triple connection: Laurie Metcalf
I also thought of Tim Allen adding to his repertoire with his new Last Man Standing. His wife on the show is played by Nancy Travis, who also qualifies with Almost Perfect and The Bill Engvall Show, and had a recurring role on Becker.
George Clooney was in
The Facts of Life (17 Eps)
and Roseanne (11 Eps)
Not a headliner in either show, but both were successful
Also, What about Ted McGinley? He was in a crapload of shows (usually just before they tanked).
Not to mention Christa Miller (“Drew Carey Show,” “Scrubs,” and “Cougar Town”) and Ian Gomez (“Drew Carey Show,” “Cougar Town”). Speaking of which, did anyone catch the recent episode that ended with about five “Scrubs” actors popping up (including J.D.)? 
I checked Busy Phillips, and it doesn’t look like she has another successful sitcom - just “Dawson’s Creek.”
Also, Lisa Kudrow has what can be considered another successful sitcom - Web Therapy. It’s been on for four years now, even though I’ve never seen it (probably channel related).