Keep your eyes peeled for a very quick Avengers cameo when you sit down to watch Captain America: The Winter Soldier in a few weeks.
No, it's not Thor or Hawkeye (although there's a very subtle shout-out to him in the movie, as well), nor is it something in the post-credits scenes. Speaking with Slashfilm, Marvel Studios president Kevin Feige revealed that Avengers Tower -- originally known as Stark Tower -- makes a very brief appearance in the film:
You get a quick glimpse, I don’t know if you noticed, of the new Stark Tower -- which is not the Stark Tower, it’s the new Avengers Tower (in Captain America: The Winter Soldier). We show it in there. It goes by real fast during that [REDACTED] sequence. But it’s there.
The reason we redacted that one small word in Feige's quote is because it gives away a potential plot spoiler for the film (the original source redacted it as well, but has also made it available for anyone who wants to know).
Stark Tower was last seen at the end of The Avengers, where it had been decimated by the Chitauri attack on New York City. Only the "A" had been left at the top, a hint for fans in the know, and a further quick scene showed Tony (Robert Downey Jr.) going over plans for what was to become Avengers Tower. Apparently those plans went forward while Tony was off chasing the Mandarin in Iron Man 3, and we'll presumably see the Tower in all its glory next year in The Avengers: Age of Ultron.
There are a number of MCU (Marvel Cinematic Universe) references in The Winter Soldier, including a very big call-out for a future movie in the same scene described above. But I'm not going to be the one to spoil it for you -- trust me, it's much more fun to go in knowing as little as possible.
Captain America: The Winter Soldier is out in theaters April 4.