UFO: Alien Invasion is a game where you must fight a brutal alien army. Build your base, prepare your team and try to destroy your enemy.
The game is based on a modified Quake2 engine.
Website: UFO: Alien Invasion
Simutrans is a game such as Transport Tycoon. You should transport gods and passagers from one destination to another. You have to build roads, railroads, tunnels, airports and buy the vehicles.
Glest is a great free strategy game in my opinion. It’s pretty similar to Warcraft. You have to build your city, create your troops and destroy your enemy. You can play against the CPU or against a friend over the network/internet.