Condition monitoring (CM) deliveries significant benefits to industries by reducing breakdown losses of machines and enhancing their safe and high-performance operations. Monitoring the machine conditions in real time using an appropriate wireless sensor network (WSN) has the advantages of the avoidance of cable usages, ease of system deployment and hence cost-effectiveness of CM implementation. One of the major challenges for WSN is the battery replacement. Generally, the batteries of sensor nodes are difficult to recharge or replace due to the inevitable layout at inaccessible or risky positions. Recently, energy harvesting (EH) applied to WSNs has increasingly caught the attention of researchers due to the ideal permanent non-maintenance requirements of the autonomous WSN nodes. This paper overviews the principles of several promising EH technologies (including photovoltaic, thermoelectric, pyroelectric, piezoelectric, electromagnetic, triboelectric EH technologies) used in various fields. In addition, the corresponding EH prototypes and fabricated products developed by various researchers are reviewed. After the discussion of the advantages and limitations of different technologies, the EH technologies are evaluated for further development of the energy harvesters to achieve a maintenance-free system for reliable monitoring machines. Finally, a discussion on challenges, applications and future developments of EH applied for machine CM is held.