MiNT ("MiNT is Now TOS") is an alternative operating system kernel for the Atari ST computer and its successors which is free software. Together with the free system components fVDI (device drivers), XaAES (GUI widgets), and TeraDesk (a file manager), you get a free TOS compatible replacement OS that is even capable of multitasking.
There are several distributions, most notably the RPM-based SpareMiNT as well as the Debian GNU/MiNT porting effort.
See also: EmuTOS
