The Avengers: Who is that Smiling at the End?

SPOILERS!! 
Who is that Smiling at end of The Avengers?

Thanos

At the end of The Marvel's 'The Avengers', int he first scene shown during the end credits, one of the ministers who had been plotting with Loki (Tom Hiddleston, War Horse) throughout the film is again seen in the recess of the other end of the universe among the floating asteroids and debris. He is talking to an unknown master about the men and women who thwarted their attempt to rule the planet Earth, stating that the protectors of the planet would continue to be problematic and that to court the rule of earth would be to court... death!

Who is Thanos?

Thanos is a god. But an Asgardian god like Thor or Loki, but a god from the race of beings known as the Eternals. The Eternal colonized Saturn's moon of Titan many millenia ago, Thanos a product of the last sons of the original colonists. Unlike his brethren, Thanos was born monstrous and misshapen. He grew to be obsessed with the concept of death. After enhancing his strength and power with cybernetic implants, he was exiled from the Eternal community and from the moon of Titan.

Traveling the universe Thanos grew in power, becoming infamous for his misdeeds. It was then, during these travels, that Thanos met the entity Death. With Death in female form, Thanos became infatuated with Death and vowed to make himself worthy of her affections. Returning to Titan, Thanos and his army nearly destroyed the entire population with Thanos claiming himself ruler. 

Thanos sought and found the cosmic cube, the Tessaract, in order to become a more powerful god. The Avengers journeyed to stop him, and were only successful after tricking Thanos into believing that the Tesseract's power was tapped out. Throwing the Tessaract away, the Avengers were able to grab the powerful object, undo what Thanos had done, and drain Thanos of the power he had gained from the Tessaract. Thanos was rescued by his starship and escaped.

Believing that Death had rejected him, Thanos then sought after the Soul Gems. One by one, he collected five of the Soul Gems and collected their power into a synthetic Soul Gem. With the power he now possessed, Thanos started extinguishing stars in the hopes that the deaths that he produced would attract Death back to him. The Avengers were again called upon to confront the mad Titan, and were only successful with the help of other cosmic beings. Once defeated, Thanos was turned into a living statue of stone, denied death altogether. The Soul Gems were distributed across the universe again.

Revived by Death herself, Thanos thought that the balance of life and death in the Universe was out of order. He went out looking for the Soul Gems again, this time under the alias of the Infinity Gems. He collected them all again and placed the gems into a gauntlet he wore as a glove. With a thought, Thanos extinguished life from one half of the universe's living beings. Confronted by most of Earth's heroes, Thanos easily toyed with and defeated them all. He only faltered when Death spurned him due to the fact that Thanos was now too powerful, allowing others to take advantage of his lapse in concentration to wrest control of the gauntlet, return the heroes and the Universe to normal and defeating Thanos again.

Thanos is a being of cosmic godlike origin, obsessed with Death and with furthering himself and his interested with as much power as he can find or muster. He is thwarted at each turn by Avengers, other of earth's heroes, heroes from across the universe, as well as abstract cosmic entities from time to time - if Thanos's powers grow too strong.

For more complete information on Thanos, read more at the Marvel Universe Wiki site, or pick up a Marvel trade paperback involving Thanos, including The Infinity Gauntlet.