Crossovers

I rewatched the Spiderman 90's cartoon recently thanks to Netflix and realized just how many crossovers there are throughout that show.  For those who don't know, a crossover is when two different IPs in the same universe intersect.  So when Spiderman and Daredevil team up to take down the Kingpin, that is a crossover.  Crossovers help to broaden the scope of a universe you've built and help make it feel much bigger than just the single entry point the consumer is enjoying.  On a business side of things, it also helps introduce people to another IP that they might start buying into now.  Maybe Daredevil seemed intriguing so they'll find out if there's a Daredevil cartoon or buy the comic.

To be done well, crossovers have to make sense (the two IPs have to exist in the same universe and time, so Naruto meeting Edward Elric is probably not going to happen believably any time soon) and they should be more than just a cameo (although that can be hilarious if done well).  So when Spiderman's mutant genes start going out of control and changing him, he goes to Charles Xavier for a cure because he has heard that's what he does.  He knows nothing about the X-Men because they are not common knowledge at that time.  So obviously when he invades the mansion, the X-Men protect him and BAM!  Crossover.  Tony Stark is showing off some amazing new technology, of course Peter Parker would be there because he's both a scientist and a news photographer.  So when someone tries to steal the technology, Iron Man and Spiderman both happen to show up?  BAM!  Crossover.

Crossovers don't necessarily need to be characters either.  An event in one IP that affects the entire universe should be brought up from time to time in the other ones because it was a big deal.  So when Deep Space 9 starts during the battle of Wolf 359 or when Voyager encounters the Borg who have some drones from the same battle, that is a well placed crossover from another series.  Of course, Voyager has its share of awesome character crossovers as well.

Crossovers are understandably hard to pull off unless one IP is a spin-off of another because both IPs have to make sense in each other's universe (Dr. Horrible will probably never be visiting McClaren's Pub since super heroes and super villains would make no sense in modern day New York).  The tones of the two shows have to mesh well together as well (the members of Sacred Heart Hospital would probably never have a murder to solve..well...actually maybe they could...).  It's a shame crossovers aren't done more frequently, but it is fairly hard to set up.  What's your favorite crossover moment?