In this paper we present BRIX, a novel modular hardware prototyping platform for applications in mobile, wearable and stationary sensing, data streaming and feedback. The system consists of three different types of compact stack- able modules, which can adapt to various applications and scenarios. The core of BRIX is a base module that contains basic motion sensors, a processor and a wireless interface. A battery module provides power for the system and makes it a mobile device. Different types of extension modules can be stacked onto the base module to extend its scope of functions by sensors, actuators and interactive elements. BRIX allows a very intuitive, inexpensive and expeditious prototyping that does not require knowledge in electronics or hardware design. In an example application, we demonstrate how BRIX can be used to track human body movements.