In the last decade, cluster computing has become the most popular high-performance computing architecture. Although numerous technological innovations have been proposed to improve the interconnection of nodes, many clusters still rely on commodity Ethernet hardware to implement message passing within parallel applications. We present Open-MX, an open-source message passing stack over generic Ethernet. It offers the same abilities as the specialized Myrinet Express stack, without requiring dedicated support from the networking hardware. Open-MX works transparently in the most popular MPI implementations through its MX interface compatibility. It also enables interoperability between hosts running the specialized MX stack and generic Ethernet hosts. We detail how Open-MX copes with the inherent limitations of the Ethernet hardware to satisfy the requirements of message passing by applying an innovative copy offload model. Combined with a careful tuning of the fabric and of the MX wire protocol, Open-MX achieves better performance than TCP implementations, especially on 10 gigabit/s hardware.