There are a ton of different items you can get in Everquest which when activated (by clicking on it) will grant you a buff. Sometimes these are buffs you can get from other players, say an item that buffs you with Haste will be known as a Haste clicky. Then you have items like the Bloodmetal Inlaid Gnollish Totem, an item that will grant you a buff that increases your AC while also stacking with every other buff in the game.
You'll find all of the different clickies I am aware of (and that are useful) listed on this page. If I am missing a clicky please leave me a comment below and I will do my best to add it to this page once I get a chance!
Bifold Focus of the Evil Eye (Rain of Fear) - Twincast clicky
Drunkard's Stein (Pub Crawl 16th Anniversary Mission) -
Food & Drink Clickies (Sprucing Up Shakey 15th Anniversary Mission) -
Ring of the Ancients (Rumor Has It... 13th Anniversary Mission) - This is one of the very few Shrink clickies in the game.
Rune of Tallon (Call of the Forsaken) - Summons a swarm pet that attacks the target for you, this pet benefits from pet AAs
Lizardscale Plated Girdle (Gates of Discord) - Haste clicky
Pegasus Feather Cloak (Classic EQ) - Levitate clicky
Mask of Vanishing (Depths of Darkhollow) - Invis clicky
Wand of Imperceptibility (Luclin) - Shrink clicky
Crumblewidget's Death Imitator (Secrets of Faydwer) - IVU clicky
Girdle of Living Thorns (Velious) - AC clicky that stacks with many other buffs
Diplomatic Papers (What Happens In Neriak... Anniversary Mission) - Absorbs 5000 damage per hit, up to 25 hits for 50000 damage.
Necromantic Dragon Bone (Dead Dragons Anniversary Mission) - Lifetaps the target for 22k damage and heals you for half
Bottled Essence of Vex (A Ranger's Tale Anniversary Mission) - Targeted AE that deals 20k damage to up to 4 targets
Miniature Horn of Unity (Defending Knowledge Anniversary Mission) - 200 HP/MP/END a tick as well as other decent boosts
Bloodmetal Inlaid Gnollish Totem (War Games Anniversary Mission) - Ac clicky. Clicking it will give you a 30 AC buff or so but you can actually upgrade the totem further for an even better buff. I think the ac it gives depends on your class Archetype but for tanks it gives 156!
Silken Trillium (Call of the Forsaken) - Increases incoming heals by 750 each
Blood Drinker's Coating (Call of the Forsaken) - 2% of the damage you deal heals you
Violet Conch of the Tempest (The Darkened Sea) - Increases your armor class + gives you a damage shield
Totem of the Wurines (Depths of Darkhollow) - DA clicky
Cursed Rage Channeler (Secrets of Faydwer) - DA clicky
Feral Guardian Channeler (Depths of Darkhollow) - DA clicky
Terror's Juju (Rain of Fear) - Reverse DS on the target (Heals you when you attack)
(Warrior) Gladiator's Plate Chestguard of War (Omens of War) - The clicky reduces the amount of damage you take, working like a mini Last Stand/Final Stand
(Shadow Knight) Duskbringer's Plate Chestguard of the Hateful (Omens of War) - Heals for you 15% of the damage done by your melee attacks
Lingering Essence of Cazic-Thule (Rain of Fear) - Can be used to Fade any enemy off your Hate list
Overhaste Clickies - Ancestral Memories Focus (Ancient Cloak of Flames, Strifetorn Necklace)
Circle of <Power,Life,Mana,Guardian> - - In Everquest the Circle of <Name> clickies work as Auras that you can click at any time with unlimited uses and give not only you but all nearby allies a benefit. They share the same restrictions other auras in the game do, so if your class already uses an aura you will overwrite it by using the Circle clicky. Also, the clickies share a 30min cooldown and last for two minutes. Those are really the only draw backs I can think of!