Knowing which of your products have never been added to an Opportunity is a good thing to know. Unfortunately, you can't get that answer with Reports, but it's easy to find out using Workbench.
You need to be on Enterprise Edition or higher, as Professional Edition and below don't have access to the API.
- go to https://workbench.developerforce.com
- click 'agree to terms' and login
- Select 'SOQL Query' in 'Jump To' picklist, leave 'Object' blank, click 'Select'
- In the 'Enter or modify a SOQL query below' box paste this
select name, productcode, id from product2 where id not in (select product2id from opportunitylineitem)
- This is the list of products that are gathering dust (good description @SteveMo :-) ).
- You can copy the results to paste somewhere else
Copy the url showing in the address bar and save it somewhere. Just
paste it back into browser at some later point and it will run the same
You can paste the same query into the Developer Console but the results are easier to copy and you can return to the query by revisiting the url when you use the Workbench option.