I always have in my backpack:
Destroy Field Rune 
Destroys fire, poison and energy fields, from any distance as long there's nothing blocking the path. This rune is useful to escape from PK traps in PvP worlds and can be used to loot monsters that may be under the fields layed by other monsters. Also needed in a couple of quests.
Disintegrate Rune 
Destroys movable objects. It can help you save you from a PK trap. You need to stand next to the item(s) you want to disintegrate. It will disintegrate up to 500 movable items (it will not disintegrate any dead human body, but it will disintegrate monster bodies). It does not count as an aggressive action, and therefore works in Protection Zones and does not prevent the player from entering them.
Amulet of Loss 
If you die in PVP with the Twist of Fate blessing and wearing an Amulet of Loss, you will only lose Twist of Fate.
It is necessary to have it in backpack on pvp worlds.
Energy Ring 
The Energy Ring provides a Magic Shield for 10 minutes.
Stone Skin Amulet 
Reduces physical and death damage by exactly 80% rounded up to the nearest whole number.
The Holy Tible 
It allow to enter in The Pits of Inferno Quest.
Complete Opticording Sphere 
It helps me to open portal in Opticording Sphere Quest
Scale of Corruption 
Allows you to use the Teleport in the room under NPC Zlak (place it in the empty coal basin before entering the Teleport), which takes you to NPC Zizzle deep under Razzachai.
Scarab Coin 
Scarab coins are decorative and also used for "sacrifices" (to activate warps) in the several Ankrahmun Tombs.
Energy Soil 
Can be used to pass the energy wall or fire wall in the Magician's Quarter Residential Area to access the Warlock and Infernalist spawns. It can also be used to enter the blue flame and access the Demons spawn under the Magician Quarter's Restricted Area.
Flask of Rust Remover 
Can be used to remove the rust from Rusty Armors, Rusty Helmets, Rusty Legs, and Rusty Shields.
Teleport Crystal 
Needed to use the Gnomish Crystal Teleport System
Spying Eye 
Item obtained in the Heart of Destruction Quest. When used, it tells you where a vortex is currently active.
Golden Prison Key 
Allows access to Prince Drazzak's
Bronze Prison Key 
Allows access to Zavarash's chamber for 1 person. Use it on in the Roshamuul Prison when standing on the respective platform to be taken to him.
Silver Prison Key 
Allows access to Terofar's chamber. Use it on in the Roshamuul Prison when standing on the respective platform to be taken to him.
All Knowing Sausages 
When used a message will appear saying where is the portal for The Great Schnitzel.
Ghostsilver Lantern 
Allow access to boss room in Forgotten Knowledge Quest.
Adventurer's Stone 
Teleports you to the Adventurer's Guild if used in a temple.
Squeezing Gear Of Girlpower 
Can be used as Rope, Shovel, Pick, Machete, Scythe and Sickle, but not as a melee weapon to destroy objects.
Blessed Wooden Stake 
Can be used on slain Vampires in order to get Vampire Dust and on dead Demons to get Demon Dust.
Obsidian Knife 
An Obsidian Knife is used to get different types of leather from the dead bodies of certain creatures, such as Minotaurs, Dragons, Dragon Lords, Lizards, Bonebeasts, Behemoths and some bosses.
Scissors 
This item allow me to cut flowers in Issavi.
Blueberry Cupcake
and Blessed Steak
It will refill your mana completely. It saved my life few times.
Wooden Key (Key:3940) 
Used to enter/exit the Fibula Dungeon.
Silver Key (Key:0909) 
The doors it opens give access to the external area of the Bastion and hence the boat that works as a shortcut between the Falcon Bastion and the underground area.
Copper Key (Key:3012) 
Used for locking and unlocking the main doors to Hellgate.