[legerdemain]

I think computer science is the university subject with the best availability of publicly available online courses.

Available courses range across the entire undergraduate curriculum from distributed systems to operating systems to cryptography. The number of different courses available within each area varies a lot, and some only have 1-2 options, but at least they are available.

The situation is much more stark in other subjects, such as mathematics. Many upper-division courses aren't available, so the best you can do is find a book for self-guided study.