This site will give you all the information you need about the Aquila Direct Connect Hub software. Aquila is available for Linux and Windows. It was written as a research project into high bandwidth, high user count servers. The Direct Connect protocol is very suitable for this because of its centralised nature.
Aquila was designed to use a reasonable amount of resources to accomplish its task as well as possible. The DC protocol is not very complex and does not require a large amount of state information. Memory consumption should therefore be minimal.
Due to the simplicity of the protocol, the main task of the server is to dispatch protocol request between users. Although this requires a lot of bandwidth, this should not use too much cpu.
The software is modularized, it can be easily extended with plugins and there are even preparations to allow extra protocol modules.
You can find the Feature List here.
You can download the software here.