Planet of the Apes -- so long as you don't look at the freaking cover art for the DVD, VHS, etc. that gives it away!!!
I also liked the ending of Unknown, although all the revelations in that movie came out of nowhere. In terms of the overall story the pieces all fit, I just would've liked it if there were clues to lead up to them instead of them just happening/appearing.
Terminator 3 -- don't know if you'd call it a twist since it was just one character misleading the rest of them, but how it ended was ballsy for a big studio flick.