Suggestions for firstline

The following packages have something in common with the package firstline. The packages are ordered in decreasing similarity.

Package extradefs: A miscellany of support macros
Package samples: A collection of Plain TeX coding examples
Package paragraphs: Paragraph tricks macros
Package toc: Table of contents macros for Plain TeX
Package parskip: Layout with zero \parindent, non-zero \parskip
Package ragged2e: Alternative versions of ragged-type commands
Package shapepar: A macro to typeset paragraphs in specific shapes
Package plain-widow: Three output routines that extend \plainoutput
Package parades: Tabulators and space between paragraphs in galley approach
Package border: Draw a black border around a Plain TeX page
Package cropmrks: Add crop marks to a Plain TeX document
Package placeins-plain: Insertions that keep their place
Package varindent: Make paragraph indentation match the length of the line above
Package window: Create windows in paragraphs
Package abspos: Absolute placement with coffins
Package acrotex-web: Establish a page layout for an on-screen (PDF) document
Package fullminipage: Minipage spanning a complete page
Package nccparskip: Modify skips between paragraphs
Package pageno: Page number-only page styles
Package wordlike: Simulating word processor layout
Package semioneside: Put only special contents on left-hand pages in two sided layout
Package chet: LaTeX layout inspired by harvmac
Package layaureo: A package to improve the A4 page layout
Package harvmac: Macros for scientific articles
Package resumemac: Plain TeX macros for resumés
Package letter1: Letter formatting macros
Package mlawriter: Write MLA style documents in Plain TeX
Package histyle: A HighStyle environment for TeX
Package automagic: Automatic counter numbering in Plain TeX
Package lextex: Typesetting legal documents using plain TeX
Package plain-grid: Four sets of macros for typesetting on a baseline grid