I always enjoy seeing shared universes among TV shows, particularly when done right. Obvious means of establishing shared worlds are through spin-offs and crossovers.
Classic examples include:
- The Six Million Dollar Man and its spin-off The Bionic Woman
- Cheers and its spin-off Frasier
- Dallas and its spin-off Knots Landing
- Buffy the Vampire Slayer and its spin-off Angel
- CSI and its spin-offs CSI: Miami and CSI: NY
The best crossovers happen when characters show up in another, totally unrelated show, firmly establishing that their shows share the same world. Classic examples of TV series that take place in the same world include:
- Magnum, P.I., Simon and Simon, and Murder, She Wrote
- Ally McBeal, Boston Public, and The Practice
- The X-Files and Millennium
- Friends and Mad About You
- Chicago Hope and Early Edition
- Everybody Loves Raymond, The Nanny, Murphy Brown, and King of Queens
- all the Law & Order series plus Homicide: Life on the Street
More recent examples of TV series that share the same world include:
- CSI, CSI: Miami, CSI: NY, Without A Trace, and Cold Case
- Grey's Anatomy and Private Practice
- Warehouse 13, Eureka, and Alphas
- Bones and The Finder
- JAG, NCIS, NCIS: Los Angeles, and Hawaii Five-0
- Scrubs and Cougar Town