What free/cheap interactive tutorials and lessons have you enjoyed?

So far, I plan to:

- Work through the Bandit box on OverTheWire.org [1]

- Talk about social engineering with the Social Engineer Toolkit [2]

- Practice CTFs with PicoCTF [3]

- ... hack the planet?

[1] https://overthewire.org/wargames/bandit/

[2] https://github.com/trustedsec/social-engineer-toolkit

[3] https://picoctf.org/