Hidden Skill Bonuses

Assassin

Blade Fury: Doesn't consume weapon durability

Blade Sentinel: Doesn't consume weapon durability

Blade Shield: Life and Mana leech apply. Consumes weapon durability

Elemental Traps: Gain's pierce of the Assassin including both claws (excluding Death Sentry)

Dual Claw Block: Faster block rate works. Improved block % and dex don't

Shadow Warrior: Dex/Str=slv x 10, skills(SWarLVL/2 + SinSkill/2) = SWarSkillLVL, rounded down.
eg. level 10 shadow warrior/2 + level 20 light sentry/2 = shadows light sentry level 15. magic item level =15+ warrior level.

Shadow Master: Dex/Str=slv x 10, skill(lv=1/2slv+1); Get's all of player's passive skills:Claw Block, Claw Mastery or OSkill Passives. magic item level =36+ Shadow Master Level.

Druid

Grizzly Bear: 10% Damage bonus to Dire Wolf and Spirit Wolf

Dire Wolf: 10% Damage to Grizzly Bear

Spirit Wolf: 25% AR Bonus to Grizzly Bear and Dire Wolf. 5% Defense bonus to Grizzly Bear and Dire Wolf

Werebear: Base attack speed increases from weapon speed and on weapon IAS. Life Regeneration +33

Werewolf: Base attack speed increases from weapon speed and on weapon IAS. Life Regeneration +25

Barbarian

-50% Block speed

Howl: 12 Missle Speed +2 Missle speed per level.  (Soft point based level 7 Double Radius level 13 Triple the Range level 19 Quadruple when compared to level 1 radius)

WW: Only weapon speed and ias on the weapon matter. (Use weapon speed calculator to determine speed.

Paladin

No longer gain passive bonus from res aura's

Prayer: Stacks with any other instance of Prayer from any source.

Cleansing: Adds reduce poison and curse duration to meditation (soft points work). Heal bonus from prayer stacks with prayers heal.

Meditation: Heal bonus stacks with prayer and cleansing heal bonus will stack with meditations heal bonus (2 auras must be running at once to stack the heal). Reduce curse/poison duration bonus from cleansing doesn't stack.


Necromancer

Blood golem: Life steal is based on monsters life not damage done.

Sorceress

Telekenisis: reduces damage to Energy Shield by 0.0625 per hard point, Meaning at 0 hard points energy shield loses 2 mana per 1 damage absorbed. At 16 hard points in TK energy shield loses 1 mana per 1 damage absorbed. At 20 hard points in TK, ES loses 0.75 mana per 1 damage absorbed

Hydra: Recieves the -res of the caster at the time of casting. Even in the sorcs -res changes after casting, the hydras doesn't.

Amazon

Decoy: Life based on your life before Max Health % is applied. , Gets 2 res per level cap 66.
Gains the properties of the enemys that hit it. eg. if General Hatestorm has mana burn, magic resistant, stone skin. When he hits your decoy, the decoy will then have the same properties, mana burn, magic resistant and stone skin. This is very handy as mana burn gives a passive 20%all res and magic resistant gives a passive 40% ele res, then stone skin gives it maxed Dr%

Valk: 2 resistance per point cap 66, magic item level =36+ Valk level, Dex/Str perlevel=10. Gains hard points in Strategy Perfection and Critical Strike

Strafe: Gains +1 arrow if you use Decoy, Strafe locks you in place, min arrows fired based on slv of spell as well as max fired.

Psn Jav : Enemies struck with your weapon will stack listed cloud/spray damage Stack together with other poison sources like rabies/psn strike etc. and most importantly Plague javelin/disease spray

Generic

Items with skill charges count as synergies as long as you don't have the point in synergies already.

Freeze duration 1/2 in Nightmare and 1/4 in Hell

Poison Duration Doubled in Hell

Credit to PureRage-DoD and Baerk.