Zero-K is a FREE, multi-platform, open source Real Time Strategy game, which aims to be a dynamic, action-packed, hassle-free experience, full of clever strategies and constantly moving combat. Includes an epic scale, realistic projectile and unit physics, terraforming terrain and hundreds of unique and interesting units.
It is best to go to the website recommended below to download the latest version of this installer. This is the installer for the Zero-K lobby, It will download and install the lobby, which when launched, will download and install all content necessary to run the game. [Still need to install Microsoft .NET 4.5.1 though]