Shos that exist in the same universe through crossover activity, please add to the list...

Friday Night Lights/Parenthood/About A Boy - Recently confirmed by EP Jason Katmis

Paul Henning Universe: Petticoat Junction/Beverly Hillbillies/Green Acres

Dukes of Hazzard/Enos/Alice - Boss Hogg appeared on an episode of Alice

Late 90s CBS Monday via crossover event: Cosby, Everybody Loves Raymond, King of Queens, Becker

Late 90s Elizabeth Taylor searches for her diamonds Monday: Murphy Brown, The Nanny, ?, ?

NBC Blackout Thursday: Mad About You/Friends/Madman of the People

Late 90s ABC Wednesday via Vegas crossover event: Ellen, Drew Carey, Coach, Grace Under Fire

Witt-Thomas-Harris Miami Saturday Night Block: Golden Girls, Empty Nest, Nurses

JAG Universe: JAG/ NCIS/ NCIS Los Angeles/ Hawaii Five-O (crossover with NCISLA)

CSI Universe: CSI/CSI Miami/CSI New York

Paramount produced NBC sitcoms: Cheers/Frasier/Wings/Tortellis

Law and Order/Homicide/Chicago Fire Universe: Homicide and all the L&O and Chicago shows

TGIF: Perfect Strangers/ Family Matters/ Step By Step/ Going Places

Fox Animation Domination: Simpsons/Futurama/Family Guy/American Dad/ Cleveland Show/ King of the Hill - through crossovers and cameos

Plus many, many shows are linked together through Detective Munch alone.

Happy Days/Laverne and Shirley/Mork and Mindy/Out of the Blue/Joanie Loves Chachi

Plus many Norman Lear shows of course.

Full House and Hangin' With Mr. Cooper

Cory Matthews from Boy Meets World sent his pal Steve Urkel a chain letter.

Steve Urkel of Family Matters flew out of the Winslow living room onto the Foster/Lambers backyard on "Step by Step".

Step by Step's Dana Foster appeared on Boy Meets World.

Eric from Boy Meets World dated Sabrina, the teenage witch while Salem also appeared on BMW.

Sabrina the teenage witch appeared on Clueless.

Clueless has the Moesha cast

The Moesha cast had the cast of The Parkers

On The Parkers, Darry Hughley appeared from "The Hughleys".

Then on the Hughleys, Nikki from The Parkers appeared on "The Hugleys".

Niecey (and other cast members )from Moesha appeared on "The Parkers"

and then Niecey appeared on "Girlfriends."

On Girlfriends, the cast of "The Game" appeared.

Las Vegas and Crossing Jordan had a couple of crossovers

Eureka and Warehouse 13 had a couple of crossovers