I'm trying to recall the most important on-again/off-again sitcom couples. I'm sure I'm missing many? This is who I have so far:

Cheers: Sam and Diane
Newhart: Michael and Stephanie
The Nanny: Maxwell and Fran
Northern Exposure: Maggie and Joel
Friends: Ross and Rachel
Frasier: Niles and Daphne
Everybody Loves Raymond: Robert and Amy
Newsradio: Dave and Lisa
How I Met Your Mother: Ted and Robin
How I Met Your Mother: Barney and Robin
The Office: Jim and Pam
The Big Bang Theory: Leonard and Penny

UK - Coupling: Steve and Susan

Impressive start!

I'd add Sam and Rebecca (Cheers) to that list

Seinfeld: Elaine and Puddy :D

Wings: Joe and Helen

The Andy Griffith Show Barney & Thelma Lou

The Brady Bunch Sam & Alice

Who’s The Boss? Tony & Angela

Saved By The Bell Zack & Kelly

227 Calvin & Brenda

Amen Ruben & Thelma

The Golden Girls Stan & Dorothy

The Golden Girls Miles & Rows

Full House Jesse & Rebecca

The Wonder Years Kevin & Winnie

Roseanne Fred & Jackie

Roseanne/The Conners David & Darlene

Fuller House Steve & DJ

the middle. Axl & Weird Ashley

the middle. Darryn & Sue

the middle. Brick & Cindy

Mary Stone and Scotty (George) - The Donna Reed Show
Dobie and Thalia - Dobie Gillis
Ellie May Clampett and Dash Riprock - The Beverly Hillbillies
Grace and (Harry Connick Jr.) - Will and Grace
Barney Fife and Juanita - Andy Griffith
Betty Jo Bradley and Eb Dawson - Petticoat Junction
Batman and Catwoman
Race Bannon and Jade - Jonny Quest

Wally & Julie Foster, Leave It To Beaver
Andy & Ellie, Andy Griffith Show
Willis & Charlene, Diff'rent Strokes
Betty & Ralph, Father Knows Best

Joanie and Chachi.

J.D. and Elliott on Scrubs. Elliott being a woman just to clarify in case anybody doesn't know, so they weren't a gay couple.

Ed.