Saddles and used Cassettes...
I have a bunch of take-off saddles that will never see the light of day.
I've also been hoarding Romin Evo's in 155mm when I can find them.
I have no idea how to determine when to replace a cassette. I've never had bad shifting problems or skipping. But after a few chain replacements I tend to swap the cassette as well. I currently have, like, 7-8x 10-speed cassettes in very messy ziplock bags.
Upgraded parts always stay around too, I think it's time to sell those off.
Every box I open in the basement seems to be a new surprise of parts!