Have you ever had a section of HTML code you wanted to use on a number of pages or forms? On a page you can't put a subform, and on a form you may not want to put a subform due to the performance hit you will get.
Provided that the section you want to add to each page/form is pure HTML (or Javascript), you can use the following technique:
-- Create a new page in the database
-- Convert your code into JavaScript (just do a number of document.write for easy conversion)
-- Paste the Javascript code into the page.
-- Save the page as myfile.js
-- Go to the pages/forms where you want to insert the code.
-- Use the following code to include the code into the page:
<script language="JavaScript" src="/database.nsf/myfile.js"></script>
That should do it.
I am using this myself on my homepage where I have a DHTML-menu at the top. Using the page with Javascript code makes it very easy to modify the menu in one place and have it show up everywhere. Each page of the site is a Notes page, with embedded views and other elements.