I just checked Fight Club's box office gross on IMDB: just over 37 mill. So I guess it could be considered a cult film.
Plenty of great flicks already mentioned: Princess Bride, Labyrinth, Dark Crystal, Reefer Madness, Eternal Sunshine, Pan's Labyrinth, Audition, Ed Wood, Mars Attacks, Plan 9, Hot Fuzz, Blade Runner, Little Shop of Horrors, Office Space, Requium for a Dream, Goonies, Monster Squad, the Ernest movies, The Prophecy, and Studio Ghibli flicks are all good movies.
Pee-Wee's Big Adventure(love it) was mentioned, but like Napolean Dynamite(and even Austin Powers), it was made on a light budget and went on to be considered a box office success. Studio Ghibli stuff is popular overseas, but not so much here. I guess what is "cult" is in the eye of the beholder. Even though it went on to be big, I consider Pulp Fiction(love it) to be somewhat cult.
I was kinda surprised to see Nuke 'em High mentioned. Fun movie, but I wouldn't say it's one of my cult faves. I'm just glad someone else out there has saw it besides me. :)
I'd also throw in Master of the Flying Guillotine and Five Deadly Venoms.
Rocketpig, my hats off to you for mentioning Caligula. I have a super-duper long cut of the movie and really enjoyed it. It's somewhat bighbrow stuff at one moment, then total trash porn the next. And it has bigshot actors in it too, which makes it even more odd. "Jupiter loves me!!!"
"I feel like I could take on the whole Empire myself."
