If the material is free and doesn't require students to sign up or give their email address, then yes, it's OK. In one of my courses I give students links to a few of my youtube videos and tell them that the material is not required for the course but is a free resource if they want to learn more about this specific topic that I mention only briefly in the lecture. However, I recommend against referring to external material too much. If the students start to think that they've paid money for a list of links to free resources, they won't be too happy 😉 But if you do it sparingly (and linking to your own created content), then I think students will appreciate the opportunity to go into more detail on a topic that might only be tangential to the course material. On the other hand, if the external content is your creation and if it's really important for the course, then it should be part of the course. The bonus lecture can be used for marketing, e.g., other courses, books, or website that require payment or sign-ups.
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