TeX is written in the programming language WEB; WEB is a tool to implement the concept of ``literate programming''.
CWEB, a WEB for C programs, written by Silvio Levy, is available as web/c_cpp/cweb
Spidery WEB supports many languages including Ada, awk, and C. It was written by Norman Ramsey and, while not in the public domain, is usable free. It is available in web/spiderweb
FWEB is a version for Fortran, Ratfor, and C written by John Krommes. It is available in web/fweb
SchemeWEB is a Unix filter that translates SchemeWEB into LaTeX source or Scheme source. It was written by John Ramsdell and is available in web/schemeweb
APLWEB is a version of WEB for APL and is available in web/apl/aplweb
FunnelWeb is a version of WEB that is language independent. It is available in web/funnelweb
Other language independent versions of WEB are nuweb (which is written in ANSI C), available in web/nuweb, and noweb, available in web/noweb
A WEB for plain TeX macro files, using noweb, has recently been made available in web/tweb