I was a Lotus Notes user some years ago, and multiple categorisation (of documents) was one its more attractive features.  As I recall, there was a drop down box to select a category, then that category was  added to the display of current categories for the document, displayed as a comma separated list.

Now I am not a Moodle developer but the latent database designer in me suggest that this changes the relationship between courses and categories from many to one to many to many, and may need a link table with courseid, category id.

I'll shut up now before I display any more ignorance

Hi Gustav, i'm very sorry for answering this late but i had to travel out of the city and was unable to connect to the web (i almost died ).

Well, to the point, my ideas for multi categories are:

1. Feature can be enabled using config.php with $CFG->coursesmulticategory
2. A table named course_category_relation holding course and category id, both are keys.
3. In creating/editing a course, replace the categories select for a multiple select (5 items or so).
4. In course listing, each one shows a comma separated list of its categories.
5. When moving a list of courses to a new category, the select again is multiple and only "adds" the selection, for deleting a course from a category you have to reach the editing for the course.

I'm testing the code now and creating the upgrade code and checking backwards compatibility and installation from scratch, once i finish that i'll post the new files, i imagine it will be well tested within next month.