Marvel's Avengers: Age of Ultron Trailer
3/4/2015: Check out the latest trailer of Avengers: Age of Ultron.
1/13/2015: Check out the trailer #2 of Avengers: Age of Ultron.
A few hours ago, a supposed trailer leak (read as well placed marketing teaser) led Marvel to release the official teaser trailer of Avengers: Age of Ultron.
Here's the synopsis via The Verge: