[jfk13]

An example worth considering is TeX, which is now 43 years old (considering only TeX82; the earlier TeX78 was a substantially different piece of software). There has been some maintenance over the years, it's true, including a few feature additions in 1990 (TeX 3.0), but I would suggest it has shown itself to be extremely durable.

[WillAdams]

At the heart of this are two wildly different technologies:

- Literate Programming which was developed so as to work around limitations of the Pascal development stack as it existed when the project was begun: http://literateprogramming.com/

- web2c which allows converting .web source into a format which may be compiled by pretty much _any_ C compiler

LP was described by Knuth as more important than TeX, but it suffers a bit from folks not understanding that it's not so much documentation (if it were, then _The TeXbook_ would be the typeset source of plain.tex) as code commentary only useful to developers working to extend/make use of the application --- there really does need to be some sort of system for manual documentation, but I suspect that it will continue to be a talented technical writer for the foreseeable future.