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.
- cropmrks: Add crop marks to a Plain TeX document
- varindent: Make paragraph indentation match the length of the line above
- thumby: Create thumb indexes for printed books
- continue: Prints ‘continuation’ marks on pages of multipage documents