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.
- cropmark-pu: Cropmark macros for Plain TeX
- gentombow: Generate Japanese-style crop marks
- thumb: Thumb marks in documents
- continue: Prints ‘continuation’ marks on pages of multipage documents