Usually when people are laughing in a horror film, it's not a good thing. Here are five prime examples.
My Bloody Valentine (1981) is a favorite of mine from waaaay back, and the cackling laugh at the end right before the credits roll is a sublime and truly special one. The example below contains the credits song as well, The Ballad of Harry Warden - which in itself is one of the best end credits tunes I may have ever heard.
While The Wizard of Oz isn't exactly horror, that damn Wicked Witch of the West sure knows how to evoke fear into the hearts of man. She is one nasty customer, and as a child I could hear that cackle in my dreams for days after the movies yearly presentation (yes, it used to be on just once a year!). "I'll get you my pretty...and your little dog too!!" Gah!!
Thriller is probably the best video of all time. And what horror fan would argue with that, with it having one of the kings of the genre making a guest voiceover at the end! Vincent Price's laugh at the very end of the Thriller video is priceless (sorry, had to!) and gave the video extra street cred with genre fans! If it's been a while since you've heard it, have a listen! (This is just the laugh - you can watch the whole video pretty much anywhere!)
Amusement is a little-seen film that while not a superior horror flick, does have a few moments of genuine creepiness (that clown!!!) - and one raving mad serial killer with a crazy-ass laugh. Take a listen:
But my favorite loony laugh comes from the hilariously over-the-top fan-favorite Evil Dead II. As Ash fights the deadites, suddenly everything becomes uproarious. As Ash watches, the deer hanging on the wall starts laughing....the the lamp....the books on the shelf.....the curtains on the window....until finally Ash busts a gut too. If you're not laughing by the end of that scene, there is something wrong with you.