Use this open source engine to build your own realtime online strategy games.
from http://www.all4you.dk/FreewareWorld
Quote
Stratagus is a free cross-platform real-time strategy gaming engine. It includes support for playing over the internet/LAN, or playing a computer opponent. The engine is configurable and can be used to create games with a wide-range of features specific to your needs. See the Games page for a list of current games using the Stratagus engine.
from http://www.all4you.dk/FreewareWorld
It's looks spiffy enough...