edX

Founded by Harvard and MIT in 2012, edX brought Ivy League education to anyone with an internet connection. Their MicroMasters programs let you complete half a master's degree online before applying to finish on campus. After 2U acquired edX in 2021 for $800 million, the free audit option got harder to find but still exists. The platform hosts courses from 160+ institutions including Berkeley, Georgetown, and the Sorbonne. Their computer science and business courses are consistently rated among the best online.