Naughty Dog released a great expansion to The Last of Us last year, when they released Left Behind. It was a prequel to the events of the story in the main game, and served to add more depth to some of the characters, and the world, that players had responded so positively to back when the game had first come out in 2013.
Now, you can experience at least Left Behind even if you never bought the base game- Naughty Dog and Sony have released the expansion as a standalone release on the PSN Store, and it is available to download right now on both, the PS3, and the PS4.
It also represents a quick and easy gateway into the great world of The Last of Us- just play the expansion, and get some idea of what all the hype is about. And when you like it (you will like it), just go ahead and purchase the base game as well.
The Last of Us, as well as its Left Behind expansion, are available exclusively on PS3 and PS4.