Nice thread.

Usual Suspects is a brilliant movie with a great plot, superb acting and a truly incredible twist.

Many a movie fan's first exposure to Kevin Spacey.

I would also recommend Se7en (If you haven't seen it aready of course!)
The ending of Se7en is not really a twist, but it's still a jawdropping scene.

Also "One Flew Over The Cuckoo's Nest", I was not expecting that ending!
Lots more - I'll post them when I've got more time.