I have an epic for you Tonight!
I love crossovers. A very enjoyable one is
When Hellmouths Collide by Kimberley Rector and Martha Wilson, It is a crossover between Hercules: the Legendary Journeys and Buffy the Vampire Slayer in the third season.
This is the authors summary: "Buffy, Xander, Giles and Willow are accidentally sent through a dimensional doorway to a strange new world, where they run into mythological heroes, Hercules and Iolaus. Together, they uncover Ethan Rayne's terrible plot against this world."
It is extremely long but it is complete. I really had not seen much of Hercules, or Xena. But it does not matter, the story works even if you not familiar with that universe.