When Microsoft announced it would move away from MS-DOS as a stand-alone operating system, Jim Hall figured it would be a good idea to start an open source project to re-implement MS-DOS so that it would continue to exist. He announced the project on June 28 1994 as PD-DOS, later renamed to Free-DOS, which even later turned into FreeDOS. FreeDOS is GPL software.
FreeDOS is actually more successful than many people realise. It's used by businesses, gamers, and it's even sold pre-installed on laptops and desktops by companies like Dell and HP. There was also a GNU/DOS distribution, but this one got discontinued in 2006.
Even though MS-DOS 6.0 and 6.22 are still available through MSDN, volume licenses, and OEM deals (it's used in embedded systems for its simplicity) FreeDOS is obviously a very good alternative for those of us who don't have MSDN accounts or enough money for volume license deals. Happy 15th, FreeDOS!