The lowest base movement speed in the game is 325. At this speed, Zeal grants 16.25 movement speed.
The movement speed stat only increases flat movement speed, and will not interact with other percentage increases.
Zeal is the cheapest item to grant increased movement speed (not counting boots). Since it also grants a small attack speed and critical strike chance bonus, it's a great item to buy for slow and immobile AD Carries such as Ashe and Varus.
This item's appearance and effects resemble the Paladin's zeal skill from Diablo II.