Vassal

VASSAL is an open-source game engine which allows users to both play in real time (over a live Internet connection) and as PBeM ( via email). It runs on all platforms, and is free for personal use.

The files that are created/used for each individual game title are known as 'mods'.

Vassal is extremely popular amongst wargamers and, of all the games engines, probably has the greatest number of games supported. Creating 'mods' is more technical than for either Cyberboard or ZunTzu.

The software can be download from the VASSAL Website.