border – Draw a black border around a Plain TeX page
The border is drawn (as it were) “on top” of the page; since the border doesn't honour the \hoffset/\voffset positioning of the text, this is less of a problem than it might be.
Maybe you are interested in the following packages as well.
- cropmarks-pt: Philip Taylor's cropmarks macros
- cropmark-pu: Cropmark macros for Plain TeX
- thumbs: Create thumb indexes
- firstline: Print the first line of a paragraph in a different font