Goober and the Trucker's Paradise [pilot only] to The Flash

Goober and the Trucker's Paradise was a failed spin off of Mayberry R.F.D. that instead ran as a one-shot special that had Goober Pyle running a truck stop.

was a spin off of The Andy Griffith Show

Gomer Pyle, U.S.M.C was a spin off of The Andy Griffith Show

The Lucy Show's Lucy Carmichael was accidentally drafted into Gomer Pyle, USMC's company on an episode of The Lucy Show.

Here's Lucy's Mary Jane Lewis was originally a character on The Lucy Show (also called The Lucille Ball Show).

Here's Lucy's Lucy Carter and Mannix's Mannix are held hostage in an episode of Here's Lucy.

Mannix returned to TV on an episode of Diagnosis Murder to solve an old unsolved case with the help of Dr Mark Sloan.

Oceanic Airlines

NCIS was a spinoff of JAG

NCIS: LA was a spinoff of NCIS

NCIS: LA and Hawaii Five-O worked together in a crossover episode to stop a smallpox outbreak

Hawaii Five-O references Dominic Alonzo from Arrow

Arrow spun off The Flash