Well, I guess we could debate "cult" classic. I usually attribute that title to a movie that did not get noticed by the viewing public in the theatre, but has developed a strong following (perhaps through DVD or VHS). Of those mentioned already, i concur with:
Any Python movie, but especially Holy Grail!!!
Dark City
Erasurehead (really disturbing)
Strange Brew (

)
5th Element (but I think it did pretty good at the box office... just bought the new special edition of the DVD.. pretty good)
City of Lost Children (loved this one).
Blade Runner
A few more I can think of:
Brazil (the uncut version seen in Canada and Europe, not the US theatrical release)
Akira (although this did well at the box office too, I think)
Plan 9 form Outer Space (just too horrible not be liked)
Brotherhood of the Wolf
Hard Boiled (early John Woo)
2001 A Space Odyessy
This is SPinal Tap (".. but this one goes to 11")
The Warriors ("...come out to playyyyyeee!")
The Adverntures of Buckaroo Bonzai ("Just remember, that no matter where you go .... there you are.")
Barberella
that's all I can think of right now....
