Backdoor - Security Platform by SDSLabs.
ctfs.me - CTF All the time
exploit-exercises - Variety of VMs to learn variety of computer security issues.
Gracker - Binary challenges having a slow learning curve, and write-ups for each level.
Hack the box - Weekly CTFs for all types of security enthusiasts.
Hack this site - Training ground for hackers.
Hacking-Lab - Ethical hacking, computer network and security challenge platform.
Hone your ninja skill - Web challenges starting from basic ones.
IO - Wargame for binary challenges.
Microcorruption - Embedded security CTF
Over the wire - Wargame maintained by OvertheWire Community
pwnable.kr - Pwn Game
Ringzer0team - Ringzer0 Team Online CTF
Root-me - Hacking and Information Security learning platform.
ROP Wargames - ROP Wargames
Smashthestack - A variety of wargames maintained by the SmashTheStack Community.
Vulnhub - VM-based for practical in digital security, computer application & network administration.
W3challs - A penetration testing training platform, which offers various computer challenges, in various categories.
Webhacking - Hacking challenges for web.
Wechall - Always online challenge site.
WTHack Online CTF - CTF Practice platform for every level of cyber security enthusiasts.