With Marvel Entertainment facing bankruptcy in the late 90s, the decision was made to sell the rights to many of its most popular superheroes. Among them were Spider-Man (Sony Pictures) along with the Fantastic Four and X-Men (20th Century Fox).
It was a decision that would come back to haunt the company and forced them to rely on characters like Captain America, Thor, and Iron Man to launch the Marvel Cinematic Universe. Luckily, that paid off for Marvel Studios and they've since regained those two superhero teams thanks to the Disney/Fox merger.
As for Spider-Man, he remains on loan from Sony, with the rival studio able to make pretty much anything they want with characters from the wall-crawler's world (Morbius and the upcoming Kraven the Hunter, for example).
However, in The Wall Street Journal's podcast, The Journal (via Bleeding Cool), Yair Landau, a former junior business executive at Sony, reveals that the studio scoffed at the chance to not only acquire Spider-Man, but every hero aside from those already owned by Fox.
"[Marvel] had just emerged out of bankruptcy. They had no cash, and they knew we had a sliver of the rights. And they also knew, obviously, that we were very interested in making Marvel movies, so they came to me, and we negotiated a deal for 25 Marvel movies for $25 million. Every single character that they controlled was on the table as part of that deal."
"The collective [Sony] team decided they didn't care about the other Marvel properties, that they didn't want to invest in them. I was told, 'Nobody gives a shit about anybody but Spider-Man.' I thought they were idiots; that's what I thought. My marching orders were to go back and just get Spider-Man."
Sony ended up paying $10 million for Spider-Man but could have had The Avengers on hand for an extra $15 million. Had that happened, there's a very good chance superhero movies wouldn't be the dominant force they are today (Avi Arad's Iron Man, for example, was going to be dressed by his toaster).
It sure is interesting to think about what might have been, though!