Answer: Atari 2600
Long before downloadable content was available on the Xbox Live network, there was downloadable content for the Atari 2600. As early as the early 1980s, CVC (Computer Video Corporation) released a solid Atari 2600 cartridge market known as GameLine.
GameLine was absolutely revolutionary for its time because the cartridge was simultaneously a modem and a reprogrammable game cartridge. Players would buy the cartridge for $ 60 (about double the cost of a regular Atari title) and then pay $ 1 per downloaded title (which would stay on the cartridge for two weeks). Although CVC had obtained licenses from multiple third-party game publishers, offered additional content, and even held high-score games to keep players engaged, the service ultimately failed to capture much of the market.
That said, founding members of CVC went on to use the networking and information expertise they had gained building GameLine to establish Quantum Computer Services. The new company created Quantum Link that would connect Commodore computers over a telephone line and eventually become America Online.