boxit – Draw a box around a text
This is pretty much the same macro that appears in the TeXbook.
Maybe you are interested in the following packages as well.
- lettermac: Simple macros for writing letters (Plain TeX)
- plain-cm: Variable sized fonts in Plain TeX
- twoside: Two-side typesetting in Plain TeX
- twocolumns: Knuth's two-column output macros