Love this book but it's surely the most off-the-wall crossover I've ever seen: Star Trek ands Here Come the Brides. If not for Mark Lenard there would be nothing linking these two television series at all.
Well, Mark Lenard, Robert Brown, and David Soul. Plus recurring appearances by Carole Shelyne, the Metron from "Arena." In the show, the heroic Bolt brothers were played by Brown, Soul, and teen idol Bobby Sherman, and Lenard's Aaron Stempel was the main villain, though he softened in the second season.