Small-size, light-weight, flexible and programmable, yet low-cost sensor node is one of the keys to enable Wireless Sensor Network (WSN) research and a wide range of user applications. In this paper, the Bracelet computer is presented. The system is designed to be flexible and wearable that it supports a range of daughter boards of user's choice, and provides different ways for the boards to be connected to the main processor board. Despite a range of features, the monetary cost of the system is kept low. An application of the system — which uses the same wireless board as the Bracelet computer for localising a running athlete — is presented in this paper to demonstrate the applicability of the system. The localisation results show that the system achieves an average positional error of 28.945cm.