Crossovers are FUN. Anybody agree with me?
Sometimes they don't end up serving either license well, though.
But what about the novel "Ishmael", which crosses over with TV's "Here Come the Brides" (which starred Mark Lenard as Aaron Stemple)? Spock is lost in time, and Aaron Stemple is able to pass him off as his nephew.
Peter David worked with Bill (Will Robinson) Mumy on DC Comics' "Return of the Worthy", which resembled a crossover with "Lost in Space", although using an alien family and their robot.
How about "Dr. Whom Meets Star Wreck"? There have been any number of parody crossovers (unlicensed) in comic form:
There was also an unlicensed book, "The Doctor and the Enterprise", a compilation of magazine instalments for which not even the author, Jean Airey, was paid.