I put free in quotes because the tradeoff is that you do research for the university as part of a graduate research assistantship (GRA). Typically the position comes with a stipend as well and usually requires being a full-time student. The officially term is "tuition remission". You may receive special consideration if you attend the same university for undergrad and grad school.

Here's more info from the school I attended: https://engineering-computer-science.wright.edu/advising-and....