Apparently just some of its benefits are lowering blood pressure, triggering the release of endorphins, reducing stress, allowing muslces to relax and it's also helpful for breathing because it empties the lungs.
It's also rumoured to be of benefit to people with cancer.
There's 4 pages of the listed benefits of laugher. I think the main benefit is laughter is the release of endorphins it causes, the bodies natural painkiller.
Laughter is the best medicine. :D Like Toby said it releases lots of endorphiners the bodies natural painkillers. But does that mean if you're in a car accident you have to burst into laughter?