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 Wdow'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.[98]
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.3 Latham was later
employed by the West Coast Avengers and built several more