Here are some resources I have found useful (:
Fundamentals:
The Magic of the Terminal: https://missing.csail.mit.edu/
SSH Tunnels: specterops guide to ssh tunnels
Linux
Bandit Wargame: https://overthewire.org/wargames/bandit
Leviathan Wargame: https://overthewire.org/wargames/leviathan
Web
Portswigger Web Academy: https://portswigger.net/web-security
Damn Vulnerable Web App: https://github.com/digininja/DVWA
Web Goat Vulnerable web app: https://owasp.org/www-project-webgoat
Mutillidae Vulnerable web app: https://github.com/webpwnized/mutillidae
Podcasts
Black Hills Information Security: https://www.youtube.com/c/BlackHillsInformationSecurity/featured
Security Weekly: https://www.youtube.com/user/SecurityWeeklyTV
Metasploit
Metasploit Unleashed Class: https://www.offensive-security.com/metasploit-unleashed
Metasploitable vulnerable target: https://github.com/rapid7/metasploitable3
Cheat Sheets:
SSH pivoting: https://sleepy-student.co/blog/ssh-tunnels
Windows File Upload: https://sleepy-student.co/blog/uploading-files-to-windows-machines