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.