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.
- knuth-letter: Knuth's example letter macros
- bellmac: Outline scheme for form letter production
- twocolumns: Knuth's two-column output macros
- resumemac: Plain TeX macros for resumés