The rapid expansion of the Internet of Things (IoT) has created a strong demand for sustainable, reliable, and maintenance-free power sources for distributed sensor nodes. Traditional battery-powered IoT systems suffer from limited lifetime, frequent replacement requirements, high maintenance costs, scalability limitations, and environmental concerns. Energy harvesting provides an attractive alternative by converting ambient sources such as mechanical vibrations, thermal gradients, and radiofrequency (RF) signals into usable electrical energy. This paper reviews key energy harvesting methods for self-powered IoT devices, including vibration-based, thermal, and RF harvesting, and discusses their operating principles, limitations, and application suitability. Since no single source can provide continuous power reliably under all conditions, multi-source and hybrid harvesting architectures are examined to improve reliability and energy availability.