Best friends tend to stick together in the acting business.As a result they play many different sets of characters over the years.The characters themselves are often best friends as well.

Here are some possible examples.

Lucille Ball & Vivian Vance
Lucy Ricardo & Ethel Mertz:I Love Lucy
Lucy Carmicheal & Vivian Bagley:The Lucy Show
Lucille Carter & Vivian Jones:Here's Lucy

Beatrice Arther & Rue McClunahan
Maude Finlay & Vivian Cavender Harmon:Maude
Dorothy Spornac & Blanche Deveraux:The Golden Girls

Andy Griffith & Don Knotts
Andy Taylor & Barney Fife:The Andy Griffith Show
Matlock & Les Calhoun:Matlock

Ron Howard & Cindy Williams
Steve & Lorrie:American Grafitti
Richie Cunningham & Shirley Feeney:Happy Days/Laverne & Shirley

Jack Lemmon & Walter Matthau
The Odd Couple
On The Front Page
Buddy Buddy
Grumpy Old Men
Out To Sea

George Burns & Jack Benny

William Shatner and Leanord Nimoy-Kirk and Spock in the STAR TRECK show and movies.
I think Ron Howard and Henry Winkler-Richie and Fonzie from HAPPY DAYS

Corey Haim and Corey Feldman from their buddy pictures.
The Friends cast
Cast from that 70's show

Jensen Ackles and Jared Padalecki (Supernatural) are best friends in real life.

Here Are 2 Good Example
1.Sherman Hemsley And Isabel Sanford were best friends in real life.
Beside All In The Family/The Jeffersons they made guest apperences together on the love boats,Lois & Clark adventures of superman and fresh prince and they did denny and old navy commercials together and they did some movies together.

2.Bill Cosby And Phylicia Rashad
they did these shows together
the Cosby show
Cosby
little bill

Jennifer Aniston and Courteney Cox
Matthew Perry and Courteney Cox
Matthew Perry and Jennifer Aniston

Carl "Alfalfa" Switzer & Tommy "Butch" Bond from the Our Gang short films were mortal enemies on screen but best friends off.

-The Who's the boss? cast
-shannon dorothy and holly marie combs (charmed)
-John Ritter and Joyce Dewitt (Three's Company)
-Pam Dawber and Rebecca Shaffer (My sister sam)

I agree The Who's The Boss Cast

According to Dick York's memoirs, his best friend on Bewitched was David White (Larry Tate). He was also very fond of Agnes Moorehead (Endora).

Over at Little House on the Prairie, Melissa Gilbert (Laura) and Alison Arngrim (Nellie) were mortal enemies onscreen, but off camera were (and still are) the best of friends.

Mary Tyler Moore and Valerie Harper were close friends, as were their MTM characters, Mary Richards and Rhoda Morgenstern.

I've read that the ensemble casts of WKRP, Barney Miller and Taxi were all very close as well. That's all I can think of.

To add to mine Holly Marie Combs and Alyssa Milano (charmed) are very close friends.

Mary Tyler Moore is also very close friends with Betty White (not that you'd ever guess that by their on-screen relationship! lol).

I wonder if The cast of Alice are Close Friends

Ron Howard and Henry Winkler (Happy Days) have always been close friends.

I've read that the ensemble cast of WKRP were all very close as well. That's all I can think of.

I concur, especially at the time Tim Reid was shooting an episode of Save Our Streets, Howard Hesseman stopped off to visit him.