Beej's Guide to C Programming: an extremely comprehensive guide to C programming. I thought I knew C very well, and I still learned a lot reading through this.

Float Exposed: a tool which visualizes the bit-representation of floating point numbers. Extremely useful for getting an intuitive understanding.

Terms of Service; Didn't Read: "“Terms of Service; Didn't Read” (short: ToS;DR) is a young project started in June 2012 to help fix the “biggest lie on the web”: almost no one really reads the terms of service we agree to all the time."