As far as the revenge-for-rape subgenre goes I SPIT ON YOUR GRAVE is one the best known (mainly thanks to that legendary poster above), but what most people who haven't seen the movie don't realize is it's actually a pretty decent movie.
A woman, who lives in NYC, travels to the country for the summer to relax and write a novel. Things go good for a few days then suddenly four local assholes (including one retard) decide to gang rape, savagely beat and humiliate the shit out of her. That's the first hour of the movie, the remaining time is her brutal revenge on the four rapists.
The budget looks to be pretty low, but director Meir Zarchi does a great job of keeping it minimalistic and it works. To me, the best thing the movie does is holding off on the violence long enough to give the victim some character. Once that happens the sudden explosion of gut wrenching violence really hits you hard. Also the length of the rape scene (nearly 30 minutes!!!) just beats the shit out of the audience senses.
If you haven't seen it then you definitely should. It's not as hardcore as you probably expect, but it's still a well-made movie and much better than the ridiculous THRILLER: A CRUEL PICTURE, which I hated. Would make a interesting triple feature with MS. 45 and NIGHT TRAIN MURDERS.
Bizarre fact: according to IMDb the main actress is the real life granddaughter of Buster Keaton!!!