Originally Posted by The Batlord

The Spider-Man movies are made by Sony. The X-Men and Fantastic Four movies are made by Fox (along with the Ben Affleck Daredevil movie). Everything else is Marvel Studios. Iron Man, Thor, Captain America, and the Avengers are all made by Marvel Studios. It's basically all down to Marvel not having their own studio when Spider-Man and X-Men became movie properties, so the rights were optioned out to Sony and Fox. But now that Marvel has their own studio they don't farm out the rights anymore and can do what they want. So long as what they want doesn't include Spider-Man or the X-Men.