CTFtime - CTF archive and place, where you can get some another CTF-related info - current overall Capture The Flag team rating, per-team statistics etc.
Vulnhub - provides materials that allows anyone to gain practical 'hands-on' experience in digital security, computer software & network administration.
HackTheBox - it's an online platform allowing you to test your penetration testing skills and exchange ideas and methodologies with other members of similar interests.
OverTheWire - it will help you learn and practice security concepts in the form of fun-filled wargames, divided by different topics like Linux exploitation, crypto, web exploitation or reverse engineering.
HackThis - discover how hacks, dumps and defacements are performed and secure your website against hackers.
CryptoPals - great way to learn about crypto - you have problems to solve which are derived from weaknesses in real-world systems and modern cryptographic constructions.
Exploit Exercises - provides a variety of virtual machines, documentation and challenges that can be used to learn about a variety of computer security issues such as privilege escalation, vulnerability analysis, exploit development, debugging, reverse engineering, and general cyber security issues.
PentestIt - virtual labs, based on real companies’ networks and containing a variety of vulnerabilities and misconfigurations.
Great resource that takes you from no 'hacking' knowledge to a level that allows you to understand many security tasks and concepts like buffer overflows.
One of the best books ever made, about security of web applications, covering mostly anything that's connected to it.
Great book for people wanting to get into bug hunting - giving you a feel of how this proccess works, where to look for flaws and much more.