i love mulholland drive - my jaw was literally on the floor after the first time i saw it, at just how clever it all was in the last 30 mins. i wasn't sure exactly what had gone on, but when you know what has, it all makes sense.
donnie darko is another obvious one - that end sequence where everything falls back into place is fantastic.
another one would be the original verison of the vanishing with its truly brilliant ending.
wim wenders until the end of the world is pretty amazing too - love the last part set in the aussie outback where they're all watching, and becoming obsessed by, their dreams on handheld sony's.
Iain