A toolkit for building custom linux systems. Strangely like the Minix with a mix of "learn by building one from small components" and "everyone has different requirements, so build your own how you want it." From the WebSite:
Linux From Scratch (LFS) is a project that provides you with the steps necessary to build your own custom Linux system.
See also: LFS, 
http://www.linuxfromscratch.org
3 pages link to LinuxFromScratch:
