Answer: Atari 2600
Long before downloadable content on the Xbox Live network, there was downloadable content for the Atari 2600. All the way back in the early 1980s, CVC (Computer Video Corporation) was marketing a beefy Atari 2600 cartridge known as GameLine.
GameLine was absolutely revolutionary for its day as the cartridge was simultaneously a modem and reprogrammable game cartridge. Players would buy the cartridge for $60 (about twice 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 secured licensing with multiple third-party game publishers, offered additional content, and even held high-score contests to keep players engaged, the service ultimately failed to capture much of the market.
That said, the founding members of CVC went on to use the networking and information expertise they had gained building GameLine to found Quantum Computer Services. The new company created Quantum Link which linked Commodore computers by phone line and would eventually go on to become America Online.