[Endoperez]

Boots of Quickness, Jade Armor and the spell Quickness all give the same effect (that of the Quickness spell). Because it's the same effect, it doesn't stack.

There are three other ways of acquiring quickness, and at least two of these stacks with the common quickness. They are the Heroic Quickness Hall of Fame ability, which increases over time, Water 9 blessing, and the Champion's Trident.
The Heroic Quickness stacks with normal quickness, as it gives a percentual ability.
Water 9 blessing gives a blessed unit 50% chance of having a second action each round. It's not sure whether this is random or deteministic (2 actions on the first turn, then 1, then 2, then 1 , then ...). This also stacks with normal quickness, AND with Heroic Quickness.
Champion's Trident either gives the normal quickness, as Jade Armor/Boots of Quickness, or yet another Quickness ability that stacks with them. I think it doesn't stack, but I'm not sure about it.