The Buy One Get One Free plugin allows you to give away one or more free items with the purchase of another item. If the customer adds a specific quantity of a product to the cart, then the plugin is able to add one or more items of the same (or another) product to the cart.
For example, give away a product for free when customers purchase 5 or more of Product A or give away an item from Category B if the customer purchases two or more items from Category A. You can customize which item you want to give away.
With this plugin you can create a promotion for a single product from the admin product interface or configure promotions for all items in a product category.