Wikipedia
HackThisSite.org, commonly referred to as HTS, is an online hacking and security website founded by Jeremy Hammond. It aims to provide users with a way to learn and practice basic and advanced "hacking" skills through a series of challenges, in a safe and legal environment. The organization has a userbase of well over 1,800,000. The actual number of active members is believed to be much lower, as most accounts are never used, or are only used shortly after creation. The most users online at the same time was 1,995 on February 5, 2012 at 2:46:10 AM CST.
HackThisSite involves a small, loose team of developers and moderators who maintain its website, IRC server, and related projects. It produces an e-zine which it releases at various hacker conventions and through its hackbloc portal. Hard copies of the zine are published by Microcosm and Quimbys. It also has a short news/blog section run by developers.