Not sure whether this has been done before (wouldn't surprise me), but It's about time a list is created so that a bored dev who stumbles across this might actually consider it.
I'll start off with a few items that i think should be stackable, and everyone can add to the list as they see fit. any reasonable or plausible suggestions i will add to list below with your name beside it.
stuff that has a range of QL's probably wouldn't happen, I'm thinking.
List:
Full DCSD's
Empty DCSD's
Notum Infusers
Fragments of the Source
Pocket boss patterns
Pocket boss crystals
boxes of ammo (yeah, i know)
Arul Saba Gems - Mireiawen
Hacker Ice-Breaker Source - Baja69
Essential Human DNA - Mireiawen
Uncle Bazzit's Generic Nano Solvent - Mireiawen
Nano Programming Interface - Mireiawen
Lead Viralbots - Mireiawen
Personal Furnace - Stigman
Empty first aid stims and treatment kits. - Misterbishop
Secured May Fly Throwing Grenade - Firezus