Batman vs Aliens vs Predators with the Joker!?!?! Yes. It exists. Sandy Collara wrote and directed what might be one of the greatest fan films ever made. The film was made for $30,000 in Northern California and was meant to be a professional calling card for Sandy as well as show how fans can know how to take good care of the source material. Kevin Smith (in 2003) has called it, "possibly the truest, best Batman movie ever made".
If you have ever read the Batman/Alien or Batman/Predator crossover comics, then this will really blow your mind!
Here is the making of featurette as well:
Someone has even made "Fan Figure" Custom Action Figures of the Fan Film itself!