UPDATE - We are getting reports that this may only be available to USA Amazon accounts, as many from Canada have had an issue. As a kind user below in the comments section has noted, the Terms of Service state that it is in fact only available to USA accounts.
Naughty Dog is allowing all North American Amazon users to "purchase" their making-of video of The Last of Us for free, entitled Grounded: Making The Last of Us. It features interviews from a wide variety of developers that helped on the project. In addition to learning a lot about the design philosophy behind The Last of Us, you get a good generalized picture of Naughty Dog's approach to games overall.
While watching, you have the opportunity to learn things like the team that made The Last of Us could have ended up making a new game in the Jak and Daxter series. In another example, the game's original idea was to not have any kind of "infected" at all, but to just have a virus that killed people people and the struggle there. And you learn where the inspiration for the infected come from.
It is an interesting view, and you have a good opportunity to understand certain design decisions around The Last of Us. More broadly, you get insight into the kind of questions that developers ask themselves as they are developing a game. For example, how different would the game have been if Ellie was Joel's daughter?
You can "purchase" it here as long as you have a North American Amazon account.
Go here to check out our review of The Last of Us.