https://www.hackthebox.eu/ http://overthewire.org/wargames/
Tools & writeups for various "Hack The Box" servers & challenges.
All the writeups are for servers, which are already decommissioned. The still active servers are password protected (password = code from root.txt file).
Also, all challenges walkthroughs are password protected. The password is the "flag" that you enter to the HTB{}