μClinux is a variation of the Linux kernel, previously maintained as a fork, that targets microcontrollers without a memory management unit (MMU). It was integrated into the mainline kernel as of 2.5.46; the project continues to develop patches and tools for microcontrollers. The homepage lists Linux kernel releases for 2.0, 2.4 and 2.6.
An iPod booting iPodLinux, based on μClinux
The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally written in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU operating system, which was written to be a free (libre) replacement for Unix.
Linus Torvalds at the LinuxCon Europe 2014 in Düsseldorf
Sankey diagram of Linux Kernel Source Lines of Code
TiVo DVR, a consumer device running Linux
An example of Linux kernel panic