Tron: The Next Day (Flynn Lives Revealed) is a 10-minute short that fills in the backstory between the first and second films. It recently appeared on YouTube and contains unused footage from Tron Legacy.

The short also bears a passing resemblance to the teaser short that was rumored to be on the Tron Legacy DVD come April, but it doesn't have any overt spoilers for Tron 3 (or TR3N or THRON or TRIN or whatever). I'm guessing Tron: The Next Day will pop up on the home release as well.

[Via Slashfilm]