Resources

LiveOverflow. Many instructional videos on exploitation and CTFing in a format very suitable for beginners.
Murmus CTF. Streams CTF challenge solving, building infrastruture for tooling, and efforts in finding new vulnerabilities in unexpected software.
GynvaelEN. Experienced security researcher who streams on topics including fuzzing, file formats, operating systems, virtual machines, PHP, and Python.
IppSec. Demonstrations of methodically penetration testing HackTheBox and VulnHub services and machines, almost as soon as they retire usually.
Derek Rook. Offensive security engineer who streams HackTheBox runs and walkthroughs.
mub1x. Penetration tester and offensive security instructor streaming HackTheBox machines on Twitch.
Justin Steven. Security researcher with several streams on CTF, binary exploitation, and interesting bugs.
OJ Reeves. Metasploit developer and security consultant with streams based on real world binary and kernel exploitation.

https://hackthebox.eu (A wide variety of vulnerable Windows, Linux, and BSD machines for users to remotely exploit, with new machines arriving every few weeks)
http://pwnable.tw/ (a newer set of high quality pwnable challenges)
http://pwnable.kr/ (one of the more popular recent wargamming sets of challenges)
https://picoctf.com/ (Designed for high school students while the event is usually new every year, it's left online and has a great difficulty progression)
https://microcorruption.com/login (one of the best interfaces, a good difficulty curve and introduction to low-level reverse engineering, specifically on an MSP430)
http://ctflearn.com/ (a new CTF based learning platform with user-contributed challenges)
http://reversing.kr/
http://hax.tor.hu/
https://w3challs.com/
https://pwn0.com/
https://io.netgarage.org/
http://ringzer0team.com/
http://www.hellboundhackers.org/
http://www.overthewire.org/wargames/
http://counterhack.net/Counter_Hack/Challenges.html
http://www.hackthissite.org/
http://vulnhub.com/
http://ctf.komodosec.com