Hello there,
Currently it is possible for Resto druids to have no cooldown on the Tranquility spell. Tranquility - Spell - World of Warcraft.
How to do this:
Get the Improved Tranquility talent Improved Tranquility - Spell - World of Warcraft.
Get the 8 set bonus of the Cenarion Raiment. Decreased Tranquility and Hurricane Cooldown - Spell - World of Warcraft.
Normally cooldowns affected by percentages don't stack. However, it does seem to stack with these 2.
Yeah, sure, it's a lvl 60 set, but woo, no CD on a massive AoE healing spell? Still yummy.