Well I've been thinking a lot about the Games Industry's parallels to the Hollywood Studio System lately, and I'm thinking about this long-list of films that never got made (I guess it's better than being recut/slaughtered by the studio brass, aka Hitman/Blade Runner/Brazil/you name it).
 Napoleon, written and directed by Stanley Kubrick.
I think Mystery Man on Film's guide through the screenplay
is good if you can't be bothered to read the screenplay for yourself. The screenplay itself is a fascinating read, and the reason I became so interested in the man.
 Heart of Darkness, written and directed by Orson Welles.
The man who brought you Citizen Kane, Touch of Evil (even if it was recut) and The Magnificent Ambersons (this was butchered) was going to adapt, for you, this classic book to the big screen, and Hollywood being Hollywood said no.
His Don Quixote looked interesting too
 Everything by Terry Gilliam.
This is just really, really sad
 Que Viva Mexico, by Sergei Eisenstein
In the same article linked in number two. Tragedy.
 Rendezvous With Rama, directed by David Fincher
Then again, this one might finally be out of Development Hell and might make screens as soon as "The Curious Case of Benjamin Button" wraps-up. I think, with Perfume and Lord of the Rings, we've finally proven that no book is unfilmable. This should be a classic.
What do you guys think? Can you add to the list?