Wikia

Reactive effects are effects which are triggered by specific events, most often by hits or blocks.

There are two different types of reactive effects:

Damage shields: These have a 100% chance to proc, but can only be triggered by melee attacks. The damage is not considered spell damage.
Reactive procs: These do not necessarily trigger off every attack. However, they are not limited to melee attacks (unless specifically mentioned otherwise). The damage inflicted is considered spell damage.

Items and Spells Providing Reactive Procs

Chest

[Demon Forged Breastplate] Equip: When struck has a 3% chance of stealing 120 life from the attacker over 4 sec. This item is crafted by a Blacksmiths with 285 skill.

Shields

[Crest of Retribution] - Equip: Deals 5 to 35 damage every time you block. This item drops off Ramstein the Gorger in Stratholme.
[Force Reactive Disk] - Equip: When the shield blocks it releases an electrical charge that damages all nearby enemies. This also has a chance of damaging the shield (deals 30-40 damage to enemies in a 10 yard radius each block). The item is crafted by Engineers with a skill level of 300.
[Petrified Lichen Guard] - Afflicts your attacker with deadly poison spores each time you block. The item can be bought from the Sporeggar Quartermaster Mycah for 15 Glowcaps, once you are honored with the Sporeggar faction.
[Vile Protector] - When struck in combat has a 1% chance of inflicting 105 to 175 Shadow damage to the attacker. Quest rewards from [38] Compendium of the Fallen.

Waist

[Girdle of Reprisal] - Equip: 1% chance when struck in combat of dealing 75 to 125 Shadow damage to the attacker. Quest reward from [43] Ledger from Tanaris^ω τ ϖ.

Trinkets

[Darkmoon Card: Vengeance] - You have a 10% chance when hit by an attack or harmful spell to deal 95 to 115 holy damage to your attacker. This item can be obtained by collecting Darkmoon Faire Furies cards and assembling them into a Furies Deck.

Felsteel Shield Spike - An enhancement that does damage when blocking. There are also other, lesser, versions of shield spikes.

Consumables

[Gift of Arthas] - Use: Increases resistance to shadow by 10. If an enemy strikes the imbiber, the attacker has a 30% chance of being inflicted with disease that increases their damage taken by 8 for 3 minutes. Lasts for 30 minutes. This item is made by Alchemists with a skill level of 240.

Mage: Molten Armor - Causes 75 Fire damage when hit. Lasts 30 min.
Mage: Frost Armor / Ice Armor - If an enemy strikes the caster, they may have their movement slowed by 30% and the time between their attacks increased by 25% for 5 sec. Lasts 30 min.
Shaman: Lightning Shield - The caster is surrounded by 3 balls of lightning. When a spell, melee or ranged attack hits the caster, the attacker will be struck for 287 Nature damage.