Once a fixture of research in the social and behavioral sciences, volunteer subjects are now only rarely used in human subjects research. Yet volunteers are a potentially valuable resource, especially for research conducted online. We argue that online volunteer laboratories are able to produce high-quality data comparable to that from other online pools. The scalability of volunteer labs means that they can produce large volumes of high-quality data for multiple researchers, while imposing little or no financial burden. Using a range of original tests, we show that volunteer and paid respondents have different motivations for participating in research, but have similar descriptive compositions. Furthermore, volunteer samples are able to replicate classic and contemporary social science findings, and produce high levels of overall response quality comparable to paid subjects. Our results suggest that online volunteer labs represent a potentially significant untapped source of human subjects data.