Kind of a random one but Coursera's Audio Signal Processing for Music Applications was a ton of fun. I had basically no exposure to either the signal processing

Kind of a random one but Coursera's Audio Signal Processing for Music Applications was a ton of fun. I had basically no exposure to either the signal processing or the music side of this course and still learned a ton. Inspired me to mess with software synthesizers as well as go back to linear algebra; opened up a whole bunch of avenues for further study.