There are 3 types of haste that stack with each other, but not themselves.
v1 is Worn haste. This will only take the highest worn haste value.
v2 is Spell haste. This is the most common form of haste. It comes from Haste potions, enchanter/shaman spells, and most bard songs (including the bard epic proc).
v1 and v2 stack up to 100% max.
v3 is Overhaste. This is from high level bard songs, and some late game clicky items. Max on this is 25%.
Max possible haste is 125%.
A good example of this is as follows:
Worn Haste (v1): Cloak of Flames @ 36% haste
Spell Haste (v2): Warsong of Zek @ 60% haste
Overhaste (v3): Call of the Muse @ 25% haste
Total, this would give you 121% haste with a max of 125%.