The open-source hardware movement is becoming increasingly popular due to the emergence of successful low-cost technologies, such as Arduino and Raspberry Pi, and thanks to the community of makers that actively share their creations to be freely studied, modified, and re-distributed. Numerous authors have proposed distinct ways to seize this approach for accomplishing a variety of learning goals: enabling scholars to explore scientific concepts, promoting students' creativity, helping them to be more fluent and expressive with new technologies, and so on. This paper reports a systematic mapping study that overviews the literature on open-source hardware in education by analyzing and classifying 676 publications. The results of our work provide: 1) guidance on the published material (identifying the most relevant papers, publication sources, institutions, and countries); 2) information about the pedagogical uses of open-source hardware (showing its main educational goals, stages, and topics where it is principally applied); and 3) directions for future research.