ChibiOS/RT is a free and efficient RTOS designed for deeply embedded applications. It offers a comprehensive set of kernel primitives and supports many architectures: ARM7, Cortex-M0, Cortex-M3, Cortex-M4, PowerPC e200z, STM8, AVR, MSP430, ColdFire, H8S, x86.
Template Em::Blocks project for the STM32F4-Discovery board as USB virtual com port. Using the standard peripheral library (not the new HAL). No board specific headers are used.
Get the driver here: http://www.st.com/web/en/catalog/tools/PF257938