This is probably the most original prank I've ever heard of a best man playing on a groom. While the bride and groom were on their honeymoon, he was watching their house. He put a pressure sensitive mat under their mattress, than can sense when the happy couple are having sex. The mat automatically tweets this information on twitter, including how long and vigorous the action was. He got most of the tech here.
You can follow the twitter feed here, if you're into text-based voyeurism. (Don't worry, the best man is remaining anonymous, and is not telling who the happy couple are.)