Sony has given permission to Disney to put Spider-Man in the MCU (Marvel Cinematic Universe). His debut will be in Captain America 3: Civil War.
Edit 1: Sony gave the rights for free. FO FREE
Edit 2: Spidy will be getting his own Marvel movie in 2017. It is unknown if he will join the Avengers.
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I doubt it being free though. Something had to be exchanged. Percentages. Rights to some other unrelated hidden cash cow. Spider-Man is just one of those characters that can attract huge flies.