The cat is out of the bag. In 2015, The Avengers will face Ultron in Avengers: Age of Ultron. The malevolent mandroid was first seen in the pages of 1968's Avengers #54. The repugnant robot was initially built by Ant-Man, but became sentient and tried to enslave the world. Ultron is a worthy foe for the Avengers and has squared off against the team onscreen a few times in the past.
Ultron first appeared in the late 1990's series The Avengers: United They Stand.
A few years later, Ultron starred in the massively underrated animated movie, Next Avengers: Heroes of Tomorrow. After defeating the original Avengers team, Ultron attempted to wipe out their super heroic children who were mentored by Tony Stark.
In the Super Hero Squad Show, Ultron made a comedic cameo as the boyfriend of Nova's robot to stop the advances of H.E.R.B.I.E.
Most recently, Ultron wreaked havoc in The Avengers: Earth's Mightiest Heroes. Initially envisioned as a prison guard for super criminals, the heroic Ultron went terribly bad before nearly enslaving mankind. Way to go Ant-Man.