letter-plain – Letter macros for Plain TeX
These macros would seem to be used by Knuth himself (a version with a slightly different default address may be found in the CTAN copy of Knuth’s “local library” of macros).
Maybe you are interested in the following packages as well.
- lettermac: Simple macros for writing letters (Plain TeX)
- toc-zar: Plain TeX table of contents macros
- harvmac: Macros for scientific articles
- lextex: Typesetting legal documents using plain TeX