Apple DOS
Daniel Terdiman (via John Gruber):
On April 10, 1978, the contract was signed. For $13,000 -- $5,200 up front, and $7,800 on delivery, and no additional royalties -- Shepardson Microsystems would build Apple’s first DOS -- and hand it over just 35 days later. […] For its money, Apple would get a file manager, an interface for integer BASIC and Applesoft BASIC, and utilities that would allow disk backup, disk recovery, and file copying.
Wikipedia has more information.