ANSWER
As of April 2001, there have been 25 movies based on previously published works by Stephen King, plus one movie based on an original screenplay by King. This answer will change, as there are at least three more movies currently in production based on his works.
The Green Mile
Apt Pupil (Different Seasons)
The Night Flier (Nightmares & Dreamscapes)
The Shawshank Redemption (Different Seasons)
Thinner (as Richard Bachman)
Dolores Claiborne
Needful Things
The Dark Half
The Mangler (Night Shift)
Sleepwalkers (original screenplay)
Misery
Graveyard Shift (Night Shift)
Pet Sematary
The Running Man (as Richard Bachman)
Stand By Me (The Body)
Maximum Overdrive (Night Shift)
Silver Bullet (Cycle of the Werewolf)
Cat's Eye (Night Shift)
Firestarter
Children of the Corn (Night Shift)
Christine
The Dead Zone
Cujo
Creepshow
The Shining
Carrie
WHO GOT IT RIGHT: Allan Christensen.
