varindent – Make paragraph indentation match the length of the line above
The package will cause paragraphs to indent to the same position as the end of the last line of the previous paragraph, unless the resulting position would take you to within 2\parindent of the end of the line.
Maybe you are interested in the following packages as well.
- border: Draw a black border around a Plain TeX page
- placeins-plain: Insertions that keep their place
- portland: Simple portrait/landscape switching
- pos-at: Position things at absolute positions on the page