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.
- ovalfbox: Boxes with frames whose corners are rounded
- tabbingbox: Save a tabbing environment in a box
- outerhbox: Collect horizontal material for contributing to a paragraph
- lettermac: Simple macros for writing letters (Plain TeX)