Quoting Joseph Rezeau in discussion http://moodle.org/mod/forum/discuss.php?d=65049

"Unfortunately there is at the moment no single user-friendly solution for sharing resources (not just files) across courses in Moodle. This is for me the most serious flaw in Moodle. I am satisfied with none of the solutions that are proposed whenever this "sharing of resources" question crops up (which is quite frequently)."

The most commonly suggested "solutions" are

Metacourses
Resources in metacourses are not transparently available to child courses
(students must first enter the metacourse to see the resources).

Copying (e.g., backup/restore or import)
It is a waste of time and resources to replicate the same file to several
courses. It is also poses a maintenance problem when such files need to
be updated. Can we all please agree that copying is not sharing?

Site files
"Files placed here can be accessed by anyone"

Using symbolic links to a common repository (e.g., MDL-9036)

In discussion http://moodle.org/mod/forum/discuss.php?d=61709#p305890
Neil Turner (a new user) asks

"So is this a design flaw in the sense that the general requirements of the end
users problem domain were not analysed well enough to identify the need for this
feature? Is it by design (for some reason I can't appreciate), is it an identified
but unfulfilled requirement, or is it an implementation problem? Is it or will it
be addressed?

"I started trying to set up Moodle 2 days ago and as a complete novice ran straight
into this issue as my first real problem. As I imagined (and this is supported by
the other posters here) I am not alone. I don't want to waste time uploading the
same file to several locations and I don't want to waste valuable web space. It
is also poses a maintenance problem when a file that has been uploaded to several
locations needs to be updated.

"Is the use of a metacourse a workaround or is it by design? At this stage I am now
beginning to doubt whether using Moodle is a good idea. I will have to start looking
around the site for information about other "frequently asked" about limitations.
Maybe this is one of very few glaring ommissions but it's disappointing that something
so obvious to the end user (ask any classroom teacher about the problems of sharing
resources) has not been identified or addressed by the system developers."