...and were/are also friends off-screen.
Leanord Nimoy & William Shatner are good friends
Larry Hagman & Patrick duffy also are. In fact, Larry talked Patrick into returning to "DALLAS" after he left.
I think Larry Hagman and Linda Gray are also good friends.
Florence Henderson was friends with Robert Reed.
Jack Klugman and Tony Randall were also good friends. In fact, Jack is the godfather of Tonys baby, and it was Tony who talked Jack into doing the "THE ODD COUPLE" reunion movie, and he gave Jack a role in a play at his dinner theater to build up Jacks confidence after his throat operation. And Jack wrote a book about their friendship, called "TONY & ME".

Tom Hanks and Peter Scolari are still friends.

ron howard and henry winkler

Joyce DeWitt and Priscilla Barnes from "Three's Company" are friends. :)

Jennifer Aniston and Courtney Cox Arquette are best friends on and off "Friends"

I think I heard or read somewhere once that Kellsey Grammer & David Hyde Peirce are friends.

Bob Newhart was good friends with Suzanne Pleshete. I remember seeing her on "THE TONIGHT SHOW" once back in the 70s, talking about "THE BOB NEWHART SHOW" and she said at one point, "....so, I also love my second husband; Bob Newhart".

Andy Griffith and Don Knotts first met on the 1958 movie "No Time for Sergeants" and remained friends for the rest of Don's life. When The
Andy Griffith Show was being cast, Don told Andy he'd need a deputy.
The rest is history. Decades later, they reunited on Matlock, in which
Don Knotts had a recurring role.

There was a very close bond between Lorne Greene, Dan Blocker and Michael Landon. When Dan
died, Lorne said it was liking losing a son and Michael said he felt like he had lost a brother. Michael
cast Dan's son Dirk in an episode of Little House on the Prairie and Lorne on his Highway to
Heaven.

Although the 1963 sitcom Harry's Girls only lasted a half-season, Susan Silo and Dawn Nickerson are still friends to this day.

Annette Funicello and Shelley Fabares first met on the Annette serial, a segment of The
Mickey Mouse Club, in 1958, and are still friends.

Valerie Bertinelli and Mackenzie Phillips of One Day at a Time would be another example. They
have taken shopping trips together.

Elizabeth Montgomery was friends with Erin & Diane Murphy. She used to take them on shopping trips, have them over to her house to play with her kids, etc.

Anson Williams & Don Most 'Happy Days'
John Ritter & Richard Kline 'Three's Company'
David L. Lander & Michael McKean 'Laverne & Shirley'

I know all of these men kept in touch after their shows were cancelled. David & Michael were friends before starring in 'Laverne & Shirley'. Richard was devestated by John's sudden death. Anson & Don still see each other often and do autograph signings together.

Paul Michael Glaser & David Soul - Starsky & Hutch (http://www.sitcomsonline.com/boards/forumdisplay.php?s=&forumid=556)