John Baez is a very active professor, not only in the university, but also online: in his webpage (one of the first "blogs" in the world wide web) you can find, literally, all sorts of things; I love to get lost surfing through it! He is an awesome instructor, I had the opportunity to attend one of his classes, MATH209A, an introduction to measure theory, and after that I decided to do my best to attend as many of his classes as I could. That's the reason that I have been attending his seminars on Network Theory and on Category Theory!
Among the topics he has covered so far, I particularly liked Lecture 2, where he explained the duality between commutative C*-algebras and Compact Hausdorff topological spaces, which are opposite of each other in the categorical sense, even if that sounds crazy. I also loved his categorical approach to Galois Theory; I really think that any mathematician that has ever studied Galois Theory using the classical approach (studying fields through groups) should have a look at the general setting were Galois Theory is defined! You can find some notes on the Seminar in his website, taken by some of the students.
Next quarter, professor Baez is offering a class on Category Theory, with a more formal approach to the field. I am looking forward to it, attending John Baez classes is always worth it!