What is Hawkeye's super power?

Hawkeye, like Natasha Romanoff aka Black Widow, is a human, except he is a lot more skilled than most. He doesn't have super powers like Captain America, Thor, or Hulk, but he is a master assassin.

He does make a cameo in Thor uncredited as Clint Barton, which I think you can say is a precursor before his role in Avengers, though he didn't have as important a role that Black Widow's character had in Iron Man 2

In case you might be curious about his skillset you will find this excerpt from Wikipedia enlightening:

While Hawkeye has no superhuman powers (with the exception of the period when using Pym particles to become Goliath), he is at the very peak of human conditioning; he is an exceptional fencer, acrobat and a grandmaster marksman, having been trained from childhood in the circus and by the criminals Trick Shot and Swordsman. This includes considerable strength, as a supervillain found out when he tried to use the superhero's 250 pounds-force (1,100 newtons) draw weight bow and found that he could not draw back the string to launch an arrow.

Hawkeye has also been thoroughly trained by Captain America in tactics, martial arts, and hand-to-hand combat. Hawkeye excels in the use of ranged weapons, especially the bow and arrow, and carries a quiver containing a number of customized "trick arrows". In his role as Ronin, Barton shows great proficiency with the katana and other melee weapons. He has gained a reputation for being able to "turn any object into a weapon", and has been seen using items such as tin plates, coins, sticks and other debris to great effect against his enemies.

Hawkeye is also known to use a "Sky-Cycle" as his mode of transportation. The Sky-Cycle is modelled after a commercial snowmobile and is fitted with anti-gravitational technology. It is voice operated and has an auto-pilot steering system. The original Sky-Cycle was custom made for Hawkeye by Jorge Latham while he was employed by Cross Technological Enterprises. Latham was later employed by the West Coast Avengers and built several more

Hawkeye is far from a filler character. He's been in every Avengers lineup in every continuity (Comics, Animated, Movie ... etc). Movie-wise, this isn't his first appearance either. He first appears in the Thor movie (the same way Black Widow appears first in Iron Man 2).

Like Black Widow, he doesn't have any super human powers and his past is shrouded in mystery (at least in the movies universe). Although one can argue that his skill with the bow and arrow is super-natural.

In the comic Ultimate Comics Hawkeye #2 there is a spot where it mentions he has higher than normal rod cells. Normal person has hundered and fifty million rod cells and hawkeye has closer to 4 hundred million. Also the muscle around his eye allow for orb distortion giving him the ability to change focal length. This could be argued as a mutation similar to those in X-Men. Which would make him a superhuman/superhero and not just a normal human.

Hawkeye has no powers at all, and as for his back story: He and his brother were orphaned at a young age and they both ran away to the circus. Hawkeye was trained by Trick-shot and Swordsman while he was at the circus, but that ended when Clint caught Swordsman counting the money that he had stolen from the circus. Swordsman asked Clint to join Him but Clint refused and Swordsman and his brother, disappointed that he gave up the opportunity, both left Clint. Later Hawkeye left the circus and saw Iron Man so he decided to fight crime as well, but was mistaken for a villain by the police. Black Widow convinced Clint that he hated Iron Man, so Clint attacked him. Afterwards Hawkeye decided he wanted to be an Avenger and attacked the mansion, and then saved it so he would look like a hero.

Hawkeye does have superhuman vision in his Ultimate Universe story line. As a result of a super soldier program, he gained enhanced vision and some other perks. However he doesn't have the same in the other Marvel universes/versions. The MCU borrows heavily from the Ultimate universe version.