MiNT

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 doing multi tasking.

There are several distributions, most notably the RPM-based SpareMiNT as well as the Debian GNU/MiNT porting effort.