"Free college" (sometimes called Promise) programs are common in U.S. higher education. Reviewing 88 studies of 25 state and local programs, I provide a nuanced picture of the mechanisms through which these programs may work and their likely effects on students, communities, and colleges. Some commonly-claimed mechanisms for these effects--e.g., improving secondary school environments or impacting residential decisions--lack empirical support or are implausible for most existing programs. Programs are consistently found to shift college-bound students to colleges where they can use more scholarship dollars, increase enrollment at eligible colleges, and (for generous local programs only) increase community school district enrollment. Less consistently, programs boost college participation and thereby degree attainment, but evidence for direct effects on college performance, persistence or completion net of enrollment is weak. There is insufficient or inconsistent evidence for program effects on secondary school performance and graduation, post-college income and debt, community population or property values, and inequality reduction according to gender, race, or socioeconomic status.