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The song from the E3 trailer is "Iron", by Woodkid. It is an amazing piece, both musically and lyrically (though they can be difficult to understand — see here) and sends chills down one's spine on repeated listenings, playing as it does over Ezio being an utter badass.
"Enough For One Life", which plays when Ezio "speaks" to Desmond through the Animus. A beautiful and haunting rendition of the game's main theme.