I second oldboy i think that was the best twist in a movie i have seen.

He Loves Me, He Loves Me Not (French movie)

Chinatown... classic film!

Frailty

I gotta say eXistenZ is a good one. A "cult" favorite.

I included this movie in my first post because it was one of my more memorable ones.
O.K., so the guy has then, you find out that it was the . Awesome! :D

John Frankenheimer's Seconds

Anything by M. Night Shyamalan ... His latest, "The Village" is centered around a plot twist. If you dont like monumentous plot twists, you won't appreciate the movie.

While most of the movies mentioned here are thrillers, my favorite plot twist movie is Bill Forsythe's scottish comedy LOCAL HERO. It's one delightful plot twist after another and every time you think you know where it's going, it takes off in a direction you hadn't anticipated. Highly recommended!

The Conversation
Angel Heart
Confidence
Dead and Buried
Heist
Spartan

I'll add a couple...

Femme Fatale (exceptionally well executed twist)

The Killing (don't know if this counts as a "twist", but it's sure a memorable shocker of an ending)

Can't leave out one of the best ever:

Witness for the Prosecution

Spanish Prisoner
The Game

Life of David Gale
The Gift

I'm sorry, but the Village has a pretty weak ending for a "suprise". Sixth Sense was good; Unbreakable was even better; but the Village just left me cold. It was obvious it was coming and when it did come, there were just too many unanswered questions. It also was trying to be clever in fooling the audience - something that the previous two films didn't do.

IMO, the Village was full of extremely stupid people who, for some reason, thought that their "really good idea" was infact, smart. Especially when it was obvious that they hadn't even thought through the consequences of their actions.

And Signs IMO didn't really have much of a twist.