I went scrounging around my garage and house and found all the mismatched screws lying around in various places, then organized them all by length and put then in labeled little drawers.
I started doing this then realized how much time and money I was wasting buying things that I already had (but couldn't find). 'Am up to about 10 cases of thing now and still have more to organize. New cases are cheap compared to what they hold.
It is a joy to be able to lay my hands on just the right fitting when I need one now.
I work on boats and have buckets of old stainless steel fasteners. About once a year I sit and organize them and put them into inventory. Very easy to make a decent hourly wage when each screw costs 25¢ or more.