Sony v. Universal
The Betamax case: in 1984 the US Supreme Court held that a technology with substantial lawful uses does not become unlawful just because it can also be used to infringe.
Universal and Disney sued Sony in 1976 because the Betamax recorder let viewers tape television, and argued Sony was liable for every recording. Lobbyist Jack Valenti told Congress in 1982 that the VCR was to the American film industry what the Boston Strangler was to the woman home alone. By five votes to four the Court found that recording a show to watch later was fair use, and that a device capable of substantial non-infringing uses does not make its maker liable.
The principle became a shield for everything from copying software to MP3 players and search engines. The irony runs both ways: home video soon became Hollywood's biggest revenue stream, and Betamax lost the format war anyway.