Same character played by the same actor on different shows.

Of course, there was the infamous final episode of Enterprise, with Jonathan Frakes and Marina Sirtis reprising their Will and Deanna characters with a voice cameo by Brent (Data) Spiner.

I haven’t seen mention above of the Ally McBeal/The Practice crossover, with characters from each show appearing on the other.

http://www.poobala.com/allyandpractice.html
Actually, that page is hosted by a site that is extremely relevant to this thread:

http://www.poobala.com/crossoverlist.html

And, in another (and incredibly ridiculous) apearance as Colonel Klink, Werner Klemperer appeared on the 1960s Batman, talking to the “wall climbing” Batman:

http://www.poobala.com/batmanandhogan.html

Thank Og I never saw this.

Way back in Post #6.

O’Brien was never a regular on TNG; he was a recurring guest. He was a regular on DS9, though. Worf was a regular on both (DS9 seasons 4-7).

Richard Grieco was Dennis Booker on 21 Jump Street and the short lived spin-off Booker.

Robert Knepper played Theodore “T-Bag” Bagwell on Prison Break. He then reprised that role for an episode of Breakout Kings

The two series have the same creators but is not really a spin-off.

Harry Morgan played Pete Porter on December Bride and its spinoff Pete and Gladys.

You might say that Majel Barret played the Computer in both TOS and TNG, but I guess one could argue it wasn’t the same computer. You can get into deep philosophical discussions about this.

JD Hogg was Flo’s uncle or something.

I hate you!:mad: I just spent the last 3 hours there. ;):D:p

She played the computer in TOS? It sounds nothing like the computer of TNG-DS9-Voy. Not saying you’re wrong, just I’ve never heard that it was her before.