stuff in:
the core code: http://lxr.linux.no/linux+v2.6.39/kernel/
the arch code for x86: http://lxr.linux.no/linux+v2.6.39/arch/x86/
some of the fs code(ext*, vfat, nfs): http://lxr.linux.no/linux+v2.6.39/fs/
the mm: http://lxr.linux.no/linux+v2.6.39/mm/
and the ipv* stacks: http://lxr.linux.no/linux+v2.6.39/net/ipv4/ , http://lxr.linux.no/linux+v2.6.39/net/ipv6/
and a few drivers from: http://lxr.linux.no/linux+v2.6.39/drivers/

i have also build my own little kernel, some time ago. it sucks, true. but it can start and print out a lot of information about the computer. (NO! it not just a custom build linux kernel, its a real os from the bottom).