Open Source PHP E-Commerce Framework: Enlight
Enlight (English version exists) is an open source framework, based on Zend and Symphony 2, for creating e-commerce applications easier and faster.
It is built by the experienced e-commerce company Shopware and also powers the base behind their commercial product.
The framework has lots of components that can bring any type of apps to life including: database connection, authentication, form handling, datagrid, navigation, e-mail delivery and more.
Besides the frontend, a base with similar features exist for the backend where both parts can be improved with the help of a plugin system (a bootstrap file is provided for creating new plugins).
Enlight is not a direct competitor to any ready-to-use e-commerce system but a starting point for anyone looking to develop custom ones.