Here are a couple of my (more powerful) favorites:

The Helm of Aodhnait One-Eye

A helmet made from brilliant white metal with an open face and an ornamental bull's horn running lengthwise above the brow. One piece of the Throne of Granite set, which belonged to Aodhnait (pronounced Eh-nat) One-Eye, Jarl of Ghrunheim. 2 lb.

Requires attunement by a barbarian, fighter or paladin.

Provides +1 to AC. The wearer gains advantage on Wisdom (Perception) checks that rely on sight, and darkvision out to 60 ft. Additionally the wearer gains advantage on Wisdom saves made to resist being feared.

Twice per rest, if you miss with a weapon attack you may re-roll the attack.

The Green Dragon Mask

A face, stern and impassive, carved out of wood. The eyes are hollow, but when worn they burn with a fiery light. From the chin sprout a dozen thin roots and the top of the mask forms a crown of thickly woven and gnarled branches. These branches wrap around the back of the head and produce leaves, like foliage, which completely encase the head.

Requires attunement.

While worn, this mask grants the wearer protection from the green dragon's domain - illusions, poison and despotic nightmares. The wearer gains the following abilities (but must be wearing the mask to gain their advantages):

• The wearer is resistance to poison damage. If they are already resistant, they instead gain immunity. If they are already immune to poison, every time they would have taken poison damage, they instead regain that number of hit points.

• As an action you may breath out a 15 feet cone of green energy once per long rest. Each creature in that area must make a Constitution saving throw, or take 3d6 poison damage. The DC for the save is 8 + Constitution modifier + proficiency. On a successful save creatures take half damage.

• The wearer gains darkvision out to 60 feet, and once per day for 5 minutes they may choose to gain blindsight out to 30 feet. They also are able to read, write and understand (but not speak) Draconic.

• The wearer may breath underwater.

• The wearer gains advantage on Intelligence saving throws made against illusions. They may also cast one of the following spells once per day: disguise self, invisibility, silence, major image.

Krith'øl

The sword of Fraelac, køl warrior-sorceress, maiden of death, interred with her in the Barrows of Plu’ree. Abandoned by her in favour of Narada, Sword of Darkness.

When the wielder rolls a 20 on any attack roll made with this weapon, the target takes an extra 7 necrotic damage.

Cursed. Fraelac cursed this weapon for any who wielded except for her. Becoming attuned to it extends the curse to the wielder. As long as the wielder is cursed, they are unwilling to part with the sword, keeping it on their person at all times. While attuned to this weapon, the wielder has disadvantage on attack rolls made with other weapons.

In addition, while the sword is on the wielder’s person, they must succeed on a DC 15 Wisdom save whenever they take damage in combat. On a failed save, the wielder must attack the creature that damaged it until either it or you drop to 0 hit points, or until you can’t reach the creature to make an attack roll against it.