Current sources of data on rental housing—such as the census or commercial databases that focus on large apartment complexes—do not reflect recent market activity or the full scope of the US rental market. To address this gap, we collected, cleaned, analyzed, mapped, and visualized eleven million Craigslist rental housing listings. The data reveal fine-grained spatial and temporal patterns within and across metropolitan housing markets in the United States. We find that some metropolitan areas have only single-digit percentages of listings below fair market rent. Nontraditional sources of volunteered geographic information offer planners real-time, local-scale estimates of rent and housing characteristics currently lacking in alternative sources, such as census data.