Not going to rehash all of the great choices already mentioned....but I never liked the ending of The Ninth Gate. It was purposely left as uncertain I guess?
SPOILERS BELOW!!! (I think)
It has been a awhile but for some reason Johnny Depp starts banging the chick, actually the chick is banging him! then she starts to turn into a demon and the gate opens up and he walks of into the mountain side toward the figure in the sky??? I might have that all messed up but I remember liking the movie alot, but getting disappointed about the ending.
No, actually he bangs the chick as the castle burns. Then Depp goes back to the Spanish book dealers (who have disappeared, maybe been murdered, as there's men moving stuff out of the shop) and discovers the missing page (the book dealing brothers counterfeited the one Frank Langella used). Then Depp apparently goes back to the castle...
What he gets out of meeting Satan? I can't remember, I've read theories on the ending. That Depp himself was the Devil and didn't know it. That the chick was the devil and wanted Depp specifically to be the one to open the gate. (I don't buy that one, she's a demon, but not the devil.)