Over the last decade, small-scale energy harvesting devices that can power household electronic systems have experienced rapid development in both the research and the industrial fields. However, the large majority of work done in this domain still focuses on high-energy-density sources, which are not always available in the vicinity of the device. In that case, it is, therefore, important to use other sources, which, nevertheless, present lower energy densities. Hence, the purpose of this paper is to investigate such harvesting methods, highlighting their differences with classical techniques that rely on high-density energy resources. Additionally, the present study also aims at reviewing existing techniques for small-scale energy harvesting using piezoelectric devices, as well as presenting new designs when dealing with low energy density sources, with a particular focus on wind and rain.