There seems to be some confusion over what 'cult' means when refering to movies. A cult movie is one which does not have mass appeal and becomes respected only amoungst a small minority of the movie watching population. Generally speaking a cult film will be disliked by most people who see it but for the few who DO like it will absolutely love it. There is no inbetween. So movies like Fight Club and The Fifth Element can't really be called cult films because they were rather popular.
I though of a few that havn't been mentioned yet;
Tron
Serenity
Lock, Stock, and Two Smoking Barrels
The Lawnmower Man