Zelda first launched in Japan on a disk system that allowed the player to save his or her progress on the disk. This posed a problem while localizing the game for the US market, whose NES didn't have a disk system accessory.
Nintendo had a groundbreaking solution: It let games save data on the cartridge via a battery-backed SRAM chip, which retains its contents after the system powered off using a constant trickle from a lithium battery. This technology, which first debuted in The Legend of Zelda for the NES in 1987, worked so well that it appeared in hundreds of games over the next decade. (Battery-backed save carts did exist before Zelda, but not with games built in. See Mini Memory for the TI-99/4A.)
(Photos by Triforce 14 and Benj Edwards)