Similar to the orphaned activities, you can create a section that contains the resources you want to use but not be immediately visible to students. Hide the entire section, but make the individual items visible in the section. You can then link to them in your assignments without students being able to see them in the course. The benefit of this is that you can see what they are easily without them being "orphaned". Of course if you have 3.3 or higher, you can just use the stealth option if it has been turned on at the site level. If not, the other methods mentioned above should still work.