Detailed notes of the course GD50 covering prerequisites, additional material and the lecture in great detail.
Some awesome data structures and algorithms that you normally won't find in textbooks or courseware.
Source code walk of the Doubly Linked List implementation of Redis. It's quite easy and I honestly learnt a bit.
Reading and annotating the code that powers Redis. I try highlighting nuggets, reason as to why certain things are done that way etc. It's very popular for having a clean and well designed code base apart from being very performant.