Smash The Stack is a wargaming network hosting multiple wargames. On their website they discuss what their network is, as well as what their wargames are about.
A Wargame in our context can be described as an ethical hacking environment that supports the simulation of real world software vulnerability theories or concepts and allows for the legal execution of exploitation techniques. Software can be an Operating System, network protocol, or any userland application.
They currently have seven different wargames, most of which are linux shell based, requiring SSH to connect. In addition to having a good selection of wargames, Smash The Stack is still updated, despite being one of the longer running wargame networks.