Udacity has around 200 courses that are completely free (but do not offer a certificate). Usually these courses are created as part of a paid micro-credential that Udacity offers called a Nanodegree. In previous years, Udacity used to release a large part of courses in a Nanodegree for free.
But in recent years, Udacity has drastically reduced the number of free courses they launch. Many of the courses that are part of newer Nanodegrees are no longer free.
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