ELKS and Fuzix: Linux – and Unix – writ very, very small
Version 0.7.0 of ELKS OS, and 0.4.0 of its creator's next baby, Fuzix, are out – if you like your 'nix systems as tiny as can be.
Neither of these OSes is a Linux distro – or even a form of Linux at all – but ELKS is related to the Linux kernel. ELKS is shorthand for the Embedded Linux Kernel Subset – it's a minimal Linux-like kernel which doesn't require a memory management unit.
Linux was originally written for Intel's 32-bit 80386 processor family, which typically sported a page-based MMU among other features. Because ELKS doesn't need that level of memory management, it's able to run on the two even older families of x86 chip that predated x86-32: the 8086 and the 80286. In its early days – the project started way back in 1995 – ELKS was called Linux-8086.