These tend to over cost some really specific abilities or be of inferior armor types and thus nobody will buy any of them. What are the useful ones?
Ones I know of:
Both:
Mithral Shirt: Weighs less than Mithral Chain Shirt by 2.5 pounds for some reason.
Luck Blade (0 Wishes): Luck Bonus and rerolls.
3.5:
Rod of Defiance (MiC): Undead have four levels less for turning
Hank's Energy Bow (Animated Series Handbook, but more likely that random mention on Wizard's website): Easy way to solve most of 3.5 archery's issues
Sunblade (EtCR): +1 Bastard Sword that counts as a light weapon and only needs short sword proficiency is worth 3000 GP.
PF:
Elven Chain, Celestial Armor, Celestial Plate Armor: Doesn't have PF's Mithral nerf. Actually are one category lighter for
all purposes.
Shield of the Mage: Unlimited arcane spells of third level and lower, albeit a random one each use. Insane out of combat. Most of the SRDs don't detail it for some reason, but
the old official one did.