From ASSS Wiki
A Small Subspace Server (ASSS) is a replacement, and not simply a clone of subgame written by grelminar. It allows a Server Administrator to customize the server by adding and removing modules, changing the moderator heirarchy, and much, much more. Additionally, ASSS is developed to be open source making it easier for additional developers to fix bugs, add features, and add custom modules. It runs on both Linux and Windows, but runs better on Linux as grelminar originally designed it solely for Linux.
It uses less system resources to run as well (unlike cpu-hogging Subgame).
For instructions on how to install the ASSS server, go to Installing ASSS.