It was formerly known as "PullThePlug.org", and is probably the oldest wargame network that is still active today.
The project currently offers 10 wargames with over 120 challenges, and should be well known amongst the challengers.
Most of the games are about lowlevel memory corruption, varying in their difficulty.
There is also a box called "bandit" available for beginners, teaching basic UNIX security, as well as web based challenges, known as the "natas" box.
You are also welcome in their irc chatroom, ircs://irc.overthewire.org:6697/#wargames, which has currently about 40 users conncted.
OverTheWire.org is brought to you by Steven, who is the first challenge site admin to implement a new type of scoring protocol, the warbox scoring via identd.
This protocol is brought to you by epoch_qwert. It allows "traditional warboxes" to be scored with a service utilising identd on the warbox and acting as a scoring proxy.
Read about using this scoring procedure in your Profile->Warboxes and in the forum.