OpenTTD - An open-source clone of the classic game ...
ANNO - ANNO is a city-building and economic simulation video game series, with real-time strategy elements.
Micropolis - Micropolis is the open source version of SimCity Classic from Maxis, developed by Will Wright and Don Hopkins.
Stronghold - Stronghold Crusader is a RTS game from the series from Stronghold.
Simutrans - Simutrans is an open-source transport simulator.
The Settlers Online - The Settlers is a free web browser based strategy game along with a gameplay that drops anywhere between Age of Empires and the online strategy style.