As I've grown older I've become more selective about what "treasures" I keep around the shop. In general, I don't throw away:
-the threaded plastic caps and plugs that come with new hydraulic cylinders, valves and other hydraulic components. I've got a coffee can half-full of those caps and plugs, and they're like gold when I'm working on hydraulic components and need to keep oil from leaking out or dirt from getting in. Metal, flat-face o-ring caps are even more precious, and should NEVER be thrown away. I once had to buy a specific set of those caps for a special repair job, and 6 of those caps cost me more than $70.
-hydraulic fittings. I never keep a fitting that leaked (duh!), but any time I replace a valve block or other component, I strip off and save all the 45-degree, 90-degree and other adapter fittings that I can salvage. Flared fittings, o-ring fittings, pipe thread fittings in every size imaginable get tossed in a 5-gallon bucket under my work bench. Someday I'm going to sort them by size and design and put them in a nice set of plastic drawers. Someday.
-wood blocks. I can't count the times I've been working in a farmer's shop and needed a 4x4, 4x6 or 6x6 wooden block, and couldn't find one. Back at the shop I've got a stack of wooden blocks that I guard from other mechanics like a dog guards a bone. A wooden block of the right size, especially an OAK wooden block, is a wonderful thing when you're trying to jack or pry heavy equipment.
-five-gallon buckets. We use a lot of oil in 5-gallon buckets at the dealership, and it breaks my heart when the boss tells me it's time to let go of my stash of empty buckets. A sturdy, empty 5-gallon bucket is hard for me to let go. That's why there's a bunch of 5-gallon buckets stacked inside each other in my garage here at home. Once I figure out a way to get them unstuck from each other, I'll be glad I saved them. If I ever find the prankster co-worker who stomped them into each other while they were loosely stacked in my truck before I brought them home...
Conversely, I have a list of things I never save:
-electrical circuit boards, relays, and wiring harnesses. When I was younger and my mind was like a steel trap, I could remember which electrical components were "good" and which ones were good only for parts. Now my memory has a hard time holding onto where I laid the hammer I was using 10 minutes ago, so it's best if I just assume I removed them from a machine for a good reason, and throw them away.
-spray paint cans that quit spraying. For some reason I keep hoping they will magically start working again. They never do.
-rusty nuts and bolts. I grew up under strict orders to never, under any circumstance, throw away a nut or bolt removed during repairs or scrapping. All nuts were to be screwed onto their respective bolt and then tossed into a 5-gallon bucket under Dad's workbench. The plan was that some winter day we'd sit down and sort them by size and condition. When Dad finally had his farm sale at the end of his career, they sold six or seven 5-gallon buckets of rusty, unsorted bolts to farmers who said they planned someday to, "sit down and sort them out."
They say, "One man's trash is another man's treasure." I try to hang on only to treasures, but...every week I fear I'm going to see myself on an episode of that TV show about hoarders.