<A HREF="http://www.cs.utah.edu/flux/oskit/"
  >The OSKit Project</A>
is a complete modular infrastructure that provides all the "bricks" (*(resource)s) needed
to easily build an *(OS), focusing on what interests you, and reusing components
for the rest.

Newbies have reportedly transformed _(Unix) programs into standalone _(OS)es in
a matter of hours, using it.