All this talk about the Grey's Anatomy/Private Practice crossover event (which ends this week) has got us at TWoP thinking: true television crossovers are rare. They are not merely spin-offs (though they often involve characters from a spin-off), and they require more than just a mere mention of something from one show on another -- actual characters from both realities must interact. For ease in production, crossovers often happen between shows on the same network -- usually between shows from the same creator -- but occasionally they occur between shows that have seemingly nothing to do with each other. So what are the best examples of this elusive and mysterious breed of television event? Here are seven that we feel stand out from the annals of character-jumping television history.

We've finally posted our list of the Sci Fi and Supernatural shows that were cancelled in their prime. Why, oh why, did they have to nix Farscape? And you don't really even want to get us started on Firefly. But check out the list and let us know if you agree or disagree with our choices, or if we missed anything.

While you are at it, see the comedies and dramas that were shut down too quickly.