Since the time I could understand what they were, I've found TV show crossovers to be fascinating. Seeing how favorite characters from different environments interact with each other is the primary draw, but there are also several "inside baseball" factors which have nerd appeal (the behind the scenes finagling to bring the crossover to reality, how the "guest" characters are written when not on "their" show, what the producers or network were trying to accomplish with the crossover, etc.).
Some of my favorite crossovers include:
Transformers/G.I. Joe – An elderly Cobra Commander appears as the villain in an episode of the Transformers cartoon. Even as a kid I knew that animation studios took a lot of shortcuts, so I thought they were just reusing the character design (and voice of Chris Latta) up until the very end of the episode when Cobra Commander lets out a "Cobra!" battle cry before disappearing into the night.
Magnum, P.I./Murder She Wrote – This was a two part crossover that began on an episode of Magnum and concluded on Murder She Wrote. Even though the Murder She Wrote episode portrays Magnum as a buffoon, it's still Magnum and therefore I still think it's awesome.
Family Matters/Step by Step – Any crossover that begins with Urkel crashing into the secondary show via a rocket pack is one you have to see.
Batman/Green Hornet – There is something quite surreal about seeing Bruce Lee, of all people, in a fight scene featuring animated KAPOWs and WHAMMOs.
And if you're wondering about the title of the post, yes there is a connection between Knight Rider and Start Trek. The Crossovers & Spin Offs Master Page at Poobala.com covers this sort of thing with unbelievable detail. Poobala is one those amazingly bizarre sites that the Internet was made for.