Deals Plugin
This plugin adds a fieldtype to allow you to set different types of deals on each product. This makes it simple to create buy one, get one free offers or other similar concepts.
Installation
To install, upload the deals folder to the craft/plugins directory. Go to settings and install the Deals product. Then add the field to the Product Types required and then you can go to each product and set up a deal.
Documentation
The description can be used on the product listing or product page, but is also displayed on the cart as an adjuster.
Please note, deals are specific to the product you set it on, not across a range of products. And while the discount is applied to the product, it only groups products by variants (so it does not match differing variants). This is the field that you will get:

Example usage code which could be used on the product listing or product template:
{% if product.productDeals is not null and product.productDeals.enabled %}
<p>{{ product.productDeals.description }}</p>
<p>
Buy {{ product.productDeals.amount }}, get
{% if product.productDeals.qtyDiscount > 0 %}
{{ product.productDeals.qtyDiscount }} free
{% elseif product.productDeals.amountDiscount > 0 %}
{{ product.productDeals.amountDiscount|currency(cart.currency) }} off
{% elseif product.productDeals.percentageDiscount > 0 %}
{{ product.productDeals.percentageDiscount }}% off
{% endif %}
</p>
{% endif %}
Support
For support or feature requests, please email me at stephen@shtc.co.uk.