Actor Clark Gregg scored a minor role as a quietly confident agent of what was eventually revealed to be S.H.I.E.L.D. in the first Iron Man partially by being a buddy of director Jon Favreau. In a classic Hollywood story of unlikely stardom, his character - Coulson - has since become a central figure to the entire Marvel Cinematic Universe as Nick Fury's field operative, appearing in all of the films except Hulk thus far and getting a big role as the "second act heavy" in Thor. In Iron Man 2, he makes reference to a "situation" in New Mexico, and after that film's credits he's seen there informing S.H.I.E.L.D. that Mjolnir has been found.
My favorite gag in this movie, incidentally, is that Thor - who, like the other Gods speaks Rennaissance Faire English - calls him "Son of Coul."
When Thor storms the S.H.I.E.L.D. base attempting to liberate Mjolnir, Coulson orders a sniper to draw a bead on him. The S.H.I.E.L.D. triggerman in question forgoes a rifle, instead grabbing a compound bow and arrows. It's a few moments later when we get a look at his face, and it's Jeremy Renner from The Hurt Locker. But Coulson calls him "Agent Barton."
This would be Clint Barton, aka "Hawkeye," another Avenger-to-be.
ITC ("in the comics"), the second "revamped" permutation of The Avengers initially consisted of Captain America leading a group of reformed bad guys: Quicksilver, Scarlet Witch (he runs fast, she does magic, they're Magneto's kids, it's complicated) and Hawkeye; who is a normal human but really, really good with a bow and arrow. This movie version, thus far, seems to more closely resemble the "Ultimate Marvel" version of the character; where he, Quicksilver and Scarlet Witch were effectively S.H.I.E.L.D's "wet work" outfit. There's some confusion as to whether those other two are allowed to appear in these movies, as Fox Studios supposedly owns the movie rights to all characters related to the X-Men franchise.
Hawkeye being a vital member of the proposed team is one of those things Avengers viewers will presumably just be asked to accept, as realistically, it doesn't make much sense that "I'm reasonably skilled with a weapon that's been obsolete since the 1400s" stacks up all that much next to "I'm the perfect physical specimen," "I'm wearing a robot," "I'm a literal god" and "I'm The Hulk" in terms of power sets.
The Incredible Hulk
When S.H.I.E.L.D. turns up to mess with Jane Foster's research, her older colleague Dr. Erik Selvig (Stellan Skarsgard) knows who they are ... and knows not to get in their way. He later explains that S.H.I.E.L.D. "disappeared" a colleague of his, who he describes as "an expert in the field of Gamma Radiation." Heh. Cute.
Selvig is almost certainly talking about Dr. Bruce Banner, aka "The Incredible Hulk;" though it's amusingly possible that he could also be referring to Samuel Sterns, last seen (in the person of actor Tim Blake Nelson) having a strange reaction to spilled gamma-irradiated sludge in the 2nd Hulk movie - the implication being that Sterns is on his way to adopting his more familiar identity as perennial Hulk antagonist "The Leader."