Widelands is an open source (GPLed) real-time strategy game.
Stratagus is a free cross-platform real-time strategy gaming engine. It includes support for playing over Internet/LAN, or playing vs. computer opponents. The engine is configurable and can be used to create games with a wide-range of features specific to your needs. It started as a clone of the realtime fantasy strategy game Warcraft II from Blizzard Entertainment, although it is used now as a game engine to build other games, sci-fi or fantasy ones. It is developed together for Windows, Linux and Linux Maemo based devices (Nokia N900).
Wargus can be used to play Warcraft II from Blizzard Entertainment.
Wyrmsun (Windows only)
Bos Wars is a futuristic real time strategy game (RTS).
Commander Stalin is a Soviet real time strategy game (RTS) based on Boswars.
Blizzard - StarCraft
Blizzard - StarCraft II