OpenTTD - An open-source clone of the classic game ...
Stronghold - Stronghold Crusader is a RTS game from the series from Stronghold.
Simutrans - Simutrans is an open-source transport simulator.
ANNO - ANNO is a city-building and economic simulation video game series, with real-time strategy elements.
Transport Tycoon Deluxe - Transport Tycoon Deluxe is a Business Simulation developed by Chris Sawyer and published by MicroProse.
Rise of Nations - Rise of Nations was launched back in 2003 but is still considered to be one of the most popular and effective real-time strategy (RTS) games of all time.