Library items which used to be associated with entities (Communication, Company, Person, Opportunity, etc.). If the communication was deleted but the library item was not deleted, then the library item has no parent record, and cannot be accessed via the user interface.

Deleting orphaned records should represent a "quick win" in terms of freeing up storage capacity.

Documents which have been uploaded to My CRM | Shared Documents.

Any images that are uploaded in the mail merge process.

Templates which have been uploaded to <My Profile> | Administration | Email and Documents | Document Templates.