The memoir class (a superset of book, report and article classes) provides a variety of what I called "page notes", which include footenotes, either at the page bottom or in the margin, or unnumbered notes in the margin which can extend for over multiple pages.

When using the article option memoir treats \chapter as \section, and \subsection as \section. and so on, so that a document initially written like a book can be typeset as an article. This was the initial idea for an easy conversion from book/report to article.