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.
- automagic: Automagic numbering in Plain TeX
- bellmac: Outline scheme for form letter production
- histyle: A "HighStyle" environment for TeX
- harvmac: Macros for scientific articles