If you hung around until after The Amazing Spider-Man 2's credits rolled then you were in for a surprise treat. At the end of it was this scene from 20th Century Fox's X-men: Days of Future Past, which is a weird bit of cross-promotion considering Spider-Man is under Sony's umbrella. The reason for it is that director Marc Webb still has a film left to complete with Fox Searchlight after (500) Days of Summer, and this was the compromise the studios made so that he could helm The Amazing Spider-Man 2.
The scene gives us a glimpse at some important characters in the X-men mythos, one being a young William Stryker, who we saw as the villain in X2, and Alex "Havok" Summers played by the returning Lucas Till from X-men: First Class. We also see a younger version of Toad and a couple of other mutants who join up alongside Jennifer Lawrence's Mystique.
X-men: Days of Future Past opens May 23rd.