This article is from the Literate Programming FAQ, by David B. Thompson email@example.com with numerous contributions by others.
Any TeX system should have it.
TeX (of course! ;-)
Distributed with TeX systems. Also avaliable in source form
Available from labrea.stanford.edu/tex/web/webman.tex
This is the original software that started it all. The original
TeX processor was written in WEB.