I think there's a cast member from every 1960's sitcoms that once appeared on I Love Lucy, The Lucy Show or Here's Lucy

My 3 Sons: William Frawley, Don Grady, Barry Livingston and Tina Cole all appeared on The Lucy Show, Fred MacMurray was on TLDCH and Beverly Garland appeared on Here's Lucy

The Addams Family: Jackie Coogan (The Lucy Show)

The Munsters: Al Lewis (Here's Lucy), Pat Priest, The Lucy Show.

Others?

I should add the 1950's and the 1970's, too.

The Andy Griffith Show - Parley Baer (I Love Lucy, Here's Lucy, The Lucy Show)

Petticoat Junction - Charles Lane (I Love Lucy, The Lucy Show, The Lucy-Desi Comedy Hour)

The Carol Burnett Show - Carol Burnett (Here's Lucy, The Lucy Show)

Alan Hale Jr., the Skipper from Gilligan's Island, appeared on both The Lucy Show and Here's Lucy.

Dick Sargent, Bewitched star (Darrin II beginning in the very late '60s), appeared on Here's Lucy.

Barbara Eden of I Dream of Jeannie had appeared on I Love Lucy. (And according to her book, Desi was chasing her with a vengeance!)

Bea Benadaret of Petticoat Junction also had appeared on I Love Lucy.

My 3 Sons: William Frawley, Don Grady, Stanley Livingston and Tina Cole all appeared on The Lucy Show, and Beverly Garland appeared on Here's Lucy
--- Also Barry Livingston ("Ernie Douglas") on The Lucy Show.

Also from Gilligan's Island, Natalie Schafer appeared on I Love Lucy.

Also from Bewitched, Sandra Gould appeared on I Love Lucy, and Mable Albertson appeared on The Lucy Show.

Also from The Andy Griffith Show, Andy Griffith and Don Knotts were on Here's Lucy.

That Girl: Lew Parker was on Here's Lucy twice, and The Lucy Show about five times. In fact, his final television appearance was on Here's Lucy.

Gomer Pyle: Jim Nabors was on The Lucy Show

The Danny Thomas Show: Danny Thomas appeared on TLDCH, The Lucy Show, and Here's Lucy. And Marjorie Lord was on TLDCH.

Dennis the Menace: Joseph Kearns was on I Love Lucy; Gale Gordon was on I Love Lucy, TLDCH, The Lucy Show and Here's Lucy; Jay North was on The Lucy Show.

The Dick Van Dyke Show: Richard Deacon was on TLDCH

The Addams Family: Jackie Coogan was on The Lucy Show twice and Here's Lucy.

I Dream of Jeannie: Besides Barbara Eden, Hayden Rorke also appeared on I Love Lucy.

Hogan's Heroes: Bob Crane and John Banner both appeared on The Lucy Show.

The Brady Bunch (barely a 1960s sitcom): Eve Plumb appeared on Here's Lucy.

The Beverly Hillbillies: Nancy Kulp appeared on I Love Lucy.

The Flying Nun: Vito Scotti (recurring role) appeared twice on The Lucy Show.

The Ghost and Mrs. Muir: Reta Shaw appeared three times on The Lucy Show and three times on Here's Lucy.

The Patty Duke Show: William Schallert appeared twice on The Lucy Show.

My Favorite Martian: Alan Hewitt appeared on The Lucy Show.

F Troop and Mayberry R.F.D.: Ken Berry appeared on The Lucy Show.

The Doris Day Show: Kaye Ballard and Jackie Joseph appeared on Here's Lucy.

The Donna Reed Show: Bob Crane (a regular for two seasons) appeared on The Lucy Show.

Julia, Hazel, and Family Affair: Can't think of anyone

Julia, Hazel, and Family Affair: Can't think of anyone

Julia - Lurene Tuttle (I Love Lucy)
Hazel - Robert Williams (I Love Lucy)
Family Affair - John Williams (The Lucy Show)

Julia - Lurene Tuttle (I Love Lucy)
Hazel - Robert Williams (I Love Lucy)
Family Affair - John Williams (The Lucy Show)

Excellent! Thanks for that. A couple more just occurred to me. I can't believe I forgot . . .

Green Acres: Eva Gabor (two times) and Eddie Albert both appeared on Here's Lucy; Sid Melton appeared three times on TLDCH; Barbara Pepper appeared many times on I Love Lucy; Eleanor Audley appeared on I Love Lucy and The Lucy Show.

Mister Ed: Larry Keating appeared on TLDCH; Leon Ames and Florence MacMichael appeared on The Lucy Show.

The Andy Griffith Show: Besides (as mentioned above) Andy Griffith and Don Knotts appearing on Here's Lucy; Parley Baer appearing on I Love Lucy, The Lucy Show, and Here's Lucy; and Jim Nabors appearing on The Lucy Show, Howard McNear appeared on I Love Lucy; Hall Smith appeared on both The Lucy Show and Here's Lucy; Paul Hartman appeared on The Lucy Show; Burt Mustin appeared on Here's Lucy; Howard Morris appeared on The Lucy Show; and Keith Thibodeaux appeared on I Love Lucy. There are probably others, as well. I'm thinking The Andy Griffith Show probably has a record number of regular cast members who also appeared on a Lucy sitcom.

Also from The Patty Duke Show, Eddie Applegate on The Lucy Show and Jean Byron on Here's Lucy.

Car 54, Where Are You? Al Lewis (Here's Lucy)

The Mothers-In-Law Kaye Ballard (Here's Lucy)

Here Come The Brides Joan Bennett (The Lucy Show)

Not a sitcom and there might be more....

Rowan & Martin's Laugh-In:

Dan Rowan -The Lucy Show
Dick Martin -The Lucy Show
Ruth Buzzi - Here's Lucy
Arte Johnson - Here's Lucy

My Mother the Car Ann Sothern (The Lucy Show), TLDCH

The Mothers-In-Law Kaye Ballard (Here's Lucy)

Also Eve Arden (I Love Lucy), Richard Deacon (TLDCH and Here's Lucy), Jerry Fogel (Here's Lucy), and in recurring roles: Desi Arnaz (I Love Lucy), Vanda Barra (The Lucy Show and Here's Lucy), and Desi Arnaz, Jr. (I Love Lucy, The Lucy Show, and Here's Lucy).

[QUOTE=biffbronson]
Dick Sargent, Bewitched star (Darrin II beginning in the very late '60s), appeared on Here's Lucy.

Another one is Mabel Albertson, who played Samantha's mother-in-law. Actually, it's seemed she played everyone's mother-in-law in the '60s and '70s! Anyway, she appeared on The Lucy Show, although not as a mother-in-law.

[QUOTE=biffbronson]
Dick Sargent, Bewitched star (Darrin II beginning in the very late '60s), appeared on Here's Lucy.

Another one is Mabel Albertson, who played Samantha's mother-in-law. Actually, it's seemed she played everyone's mother-in-law in the '60s and '70s! Anyway, she appeared on The Lucy Show, although not as a mother-in-law.
...and Mr. Stephens (Roy Roberts) played Mr. Cheever On The Lucy Show and Sandra Gould ( Mrs. Kravitz (2)) was on I Love Lucy and The Lucy Show.

I don't doubt it. For a chance to work with theee Lucille Ball, I bet many of them would have done it for nothing.