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.
Hacking: The Art of Exploitation - 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.
The Web Application Hacker’s Handbook: Finding and Exploiting Security Flaws - one of the best books ever made, about security of web applications, covering mostly anything that’s connected to it.
A Bug Hunter’s Diary: A Guided Tour Through the Wilds of Software Security - 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.