Capture The Flag (CTF)

https://defcongroups.org/

https://www.pentesterlab.com

http://www.itsecgames.com

http://www.enigmagroup.org

http://smashthestack.org

http://3564020356.org

http://0x0539.net

https://www.root-me.org – webclient and webserver challenges can be done on the go, Capture The Flag, etc.

https://www.hackthebox.eu

https://www.hackthissite.org

http://overthewire.org

https://picoctf.com

https://www.vulnhub.com

http://www.dvwa.co.uk

https://www.hackerrank.com/ – Very good for learning programming, has on-the-go exercises

hacksplaining.com