Linux kernel sure is a gr8 miracle as all of it comes from voluntary work although there are BIG contributions from Companies like RedHat,IBM etc.got lotta arch support!
ppl needs to consider comparing with FreeBSD kernel
afaik the most hardened and stable one powering many prominent web sites!but who cares Linux kernel steals the show
.apple uses(looted) FreeBSD as core for MacOS X as these BSD guys licenced the kernel with BSD licence which allows copying without retaining original source work unlike Linux's GPL.