Suggestions for letter1
The following packages have something in common with the package letter1. The packages are ordered in decreasing similarity.
- lettermac: Simple macros for writing letters (Plain TeX)
 - letter-plain: Letter macros for Plain TeX
 - automagic: Automagic numbering in Plain TeX
 - harvmac: Macros for scientific articles
 - lextex: Typesetting legal documents using plain TeX
 - resumemac: Plain TeX macros for resumés
 - mlawriter: Write MLA style documents in Plain TeX
 - histyle: A “HighStyle” environment for TeX
 - plain-widow: Three output routines that extend \plainoutput
 - ifis-macros: Check if a given input string is a number or dimension for TeX
 - fontch: Changing fonts, sizes and encodings in Plain TeX
 - figplace: Floating figures in Plain TeX
 - font-selection: Font selection for plain TeX
 - font-change: Macros to change text and mathematics fonts in plain TeX
 - hyplain: Basic support for multiple languages in Plain TeX
 - list: List ASCII text files
 - label-pln: Print address labels in three columns
 - pdcmac: Damian Cugley's document tools
 - twoside: Two-side typesetting in Plain TeX
 - toc: Table of contents macros for Plain TeX
 - twocolumns: Knuth's two-column output macros
 - toc-zar: Plain TeX table of contents macros
 - bellmac: Outline scheme for form letter production
 - make-env: Print USPS standard envelopes
 - knuth-letter: Knuth’s example letter macros
 - eplain: Extended plain TeX macros
 - twoinone: Print two pages on a single page
 - plnfss: Font selection for Plain TeX
 - graphics-pln: LaTeX-style graphics for Plain TeX users
 - ofs: Macros for managing large font collections
 - knst: Multiple-guess tests