It's tame by today's standards but I saw this in the late '70s when I was twelve and I almost swore off horror movies altogether. This movie actually paved the way for "Halloween" and all the subsequent "slasher flicks". It was also (as far as I know) the first of the "The killer is IN THE HOUSE!!!"-movies where a character finds out that the killer is not "out there" somewhere but has been lurking in the house the whole time. The killer also makes some very unsettling obscene phone calls which all by themselves will send a chill up your spine. Definitely check this one out.
http://www.imdb.com/title/tt0071222/