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
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.