Melrose Place (2009) to Scorpion

Melrose Place (2009) was a continuation of Melrose Place which was a spin off of Beverly Hills 90210 that was continued as 90210

90210 was a spun off of Beverly Hills 90210

Brenda Walsh picks up a Morley smoking habit while in Paris on Beverly Hills 90210

Homicide's John Munch questioned The X-Files' The Lone Gunmen.

Law and Order and Homicide crossed over on four episodes

Hudson University is the fictional University where many Law & Order characters graduated as well as the location of F.I.R.E.S.T.O.R.M. On Arrow

Hawaii Five-O references Dominic Alonzo from Arrow

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

Hetty Lange from NCIS: LA helped out the investigators from Scorpion on a case