General:
* Postscript (often abbreviated as "P.S.") - This is the most common and formal option, usually used at the end of a letter or document.
* Addendum - This implies a more formal addition to a document.
* Afterthought - This suggests the note was written after the main text was completed, often because something was forgotten.
* Later note - This is a simple and straightforward option.
More specific:
* Appendix - Used for a supplemental section added to the end of a book or report.
* Footnote - Used for a brief explanation or reference added at the bottom of a page.
* Endnote - Similar to a footnote but placed at the end of a document.
* Amendment - Used to indicate a change or correction added to a document after it was originally written.
* Codicil - This is a formal addition to a will, often used to make changes or additions to the original document.
Informal:
* P.S. - (See above) - This can be used informally as well.
* Afterward - This is a general term meaning "after the event" and could be used to describe the note.
* Second thought - This implies that the writer had an afterthought about something and wanted to add it in.
The best word to use will depend on the context and the purpose of the note.