The "Copy" function in the Resource Library allows you to have a single Document appear in multiple folders (you can even set different permissions for each copy).
Simply select the "Copy" link in the Document dropdown and follow the instructions.
You can view all linked copies of a Document on the Document Details page, accessible via the context menu:
When you edit a Document that exists in more than one location, you can specify whether to edit all locations or even separate the files completely:
