Answer: Napster

As long as there have been computer networks, there have been ways for people to share files. In the early days, however, sharing files was the sole province of the computer savvy. The general public wasn’t dialing into “warez” bulletin board systems and downloading ebooks, games, and music files (in the early days, bandwidth constraints put everything but simple images, text documents, and MIDI files beyond the reach of home users).

The public awareness of the concept of file sharing and the actual use of such all changed radically in 1999 when the music sharing service Napster burst into the public sphere.

Although certainly limited by today’s standards (you could only trade music files), Napster was the public’s first taste of peer-to-peer file sharing. Unlike the more esoteric and difficult to use methods that relied on downloading and reassembling file binaries from Usenet or dialing into private servers, people could just download Napster, start searching for MP3 files, and via the Napster server system, they would be connected with someone somewhere else in the world that had the file they were looking for. Almost overnight it became trivial for the average person to swap files.

While quite popular, Napster was quickly sued out of existence thanks to its centralized server model (later returning as an identically named but different music service that wasn’t focused on illegal file sharing). It was quickly replaced by a flood of decentralized apps like Bearshare, Kazaa, and Gnutella. Eventually those apps in turn were replaced with the rise of the BitTorrent file sharing protocol, long after the file sharing genie was out of the bottle.