Retail store project I'm currently involved with heavily relies on Magento catalog price rules to offer pre-cart discounts on selected articles. Even though Magento CatalogRule module does most heavy lifting when it comes to placing products on sale, code allowing logical organization and filtering for sale products at store frontend is left to the solution provider's imagination. In this article I'll cover the filtering part of this scenario, specifically how to make it possible for customer to filter sale and not on sale products while browsing category view pages. There are couple of ways to tackle this problem, with creating custom layered navigation filter being probably the most logical approach.