Epic Movie made me cry because it was so bad.
Seriously though, I don't think I've cried because of a film but.. Titanic, The Pianist and maybe even The Great Escape are probably the closest I've come. I can't remember off the top of my head but I'm sure there are some more.
Pursuit of Happyness, possibly...
I'm sure Robin Williams has made me upset in some film. I'm about to watch One Hour Photo on Channel 4, maybe it's that.
I think if any other actor had played the main character in The Truman Show then that would have made me emotional but I just can't take Jim Carrey seriously because of certain films he's been in.