This plugin alllows you to give a free product to a customer when they reach a certain dollar amount in their purchase. This amount is hard-coded, but can be modified. The dollar amount excludes shipping and taxes. The free gift is controlled from within the AbleCommerce administration. The free product is added to the customer cart at checkout.
For example, if you want a customer to have to spend $100 on qualifying products in order to get the "free" t-shirt product, you set the minimum value to $100. When your customer has has added $100 or more of product to their cart, when they go to the checkout, the product will automatically appear in their cart.
To see the plugin in action, visit:
Olive Nation
Add $50 or more of any product to your cart, and go to the checkout.