The Onion AV Club recently had an entertaining article on TV spinoffs and the various categories thereof. But for the real lowdown on spinoffs, you've got to go to The Crossovers and Spinoffs Master Page, which details just about every spinoff and inter-show crossover the author can find. Drawing on that page, Dwayne McDuffie used the many St. Elsewhere crossovers to prove that almost every show ever made was all part of that kid's dream in the St. Elsewhere finale (because if St. Elsewhere crossed over with Cheers, then Cheers never happened, which means Frasier never happened, etc).
One thing that's kind of fun -- well, depending on your definition of fun -- about watching reruns of some '70s and '80s shows is that sometimes you might come across a pilot for a spinoff. The cheapest way for a production company to do a pilot was to produce it as part of an existing show. So you'd get an episode of your favorite show where the main character would appear once at the beginning and once more at the end, and the rest of the episode would be devoted to new actors interacting on a new set. I seem to recall a Matlock (which I watched as a child, and I can't even blame my grandparents for that; I just liked to see Andy outwit those young punks) that was a spinoff pilot where an old, out-of-shape, possibly ill George Peppard teamed up with a young female who may or may not have been his daughter -- I can't remember -- to solve mysteries. Even I knew that wasn't going to fly.
When it comes to the most common type of spinoff, namely taking a supporting player and making him or her the lead, the question is which show suffered the most for spinning off a character. I don't think The Mary Tyler Moore Show suffered too badly for losing Rhoda; all it meant was that most of the stories wound up taking place in the newsroom, which was the most interesting part of the show anyway. And the third season of Soap, the first without Benson, may have been the best; Benson was an expendable character, funny as he was, because he never actually participated in any soap-opera stories of his own, being too sensible to get into that kind of trouble. (By the way, if you have the third season DVD of Soap you'll notice that Benson appears in the first three or so episodes, and they don't hire a replacement right away; the producers sensibly left the door open for him to come back to the show if his spinoff didn't catch on.) I'd say that losing Lionel Jefferson was a big blow to All in the Family. His relationship with Archie -- Lionel quietly deflating Archie with sarcasm, but understanding and liking Archie better than Mike did -- was one of the best and funniest on the show, and his way of dealing with Archie was an antidote to the self-righteous Meathead.