Meet the NeuroStimDuino: an open source neurostimulator based on Arduino for students and researchers interested in this science. It has an I2C interface for external microcontrollers. Each shield has 2 output channels to simultaneously contract multiple muscles.
Thanks to NeuroStimDuino, you can generate biphasic, rectangular and constant current stimulation pulses with programmable frequency, pulse-width, and amplitude. These are stackable and can be used with a 9V battery. On-board LEDs are available to visualize activity.