Xbox announced an Xbox Backward Compatibility Sale which starts today and runs through May 27th. You can find deals of up to 75% off on nearly 200 games on the Microsoft website. Popular games, such as Red Dead Redemption and Call of Duty: Modern Warfare 3, are included in the sale.
This sale comes on the same day that Sony announced that PlayStation 5 players would be able to play with PlayStation 4 players using backwards compatibility and an earlier announce that the new console would also be backwards compatible with PlayStation 3 game. Given the lengths Microsoft went to to ensure the Xbox One was backwards compatible with the Xbox 360 during this generation, we can probably expect the next generation of Xbox consoles to be backwards compatible as well, so it would probably be wise to take advantage of the sale while it is available.