Sins of a Solar Empire is a science fiction Real-Time Strategy (RTS) game. You're in control of a space empire must conquer star systems with military, economy and diplomacy.
Space Engineers is a sandbox building and survival game set in space. With emphasis on engineering, construction and maintenance, players build space ships and stations, pilot ships and mine asteroids for resources. Players can also commandeer and pillage neutral "cargo ships" for extra resources. Meteorite showers can also pose a threat to anything in their path, especially without proper defenses (should the option be enabled).
"Star Conflict is an action-packed, massively multiplayer space simulation game that puts players in the role of elite pilots engaging in a widespread interplanetary skirmish. Three thousand years have passed since the first colonists left Earth. Now the galaxy is divided between the militant star empires and independent mercenary groups." Source: http://store.steampowered.com/app/212070
"StarForge is a game about gathering resources, building bases, crafting, and surviving on an alien planet. Earth's star is dying and humanity transferred as much technology and resources as possible and left on a one way mission to populate another planet. It borrows elements from the Survival, FPS, RTS, RPG, Voxel Builder, and Physics Sandbox genres." - Source: StarForge.com
This game is like a side-scrolling action, adventure and sandbox RPG. Terraria is complete with resource gathering and monster killing, building and destruction as well as strategy/city-building included. Terraria has multiplayer (including PvP) available.
A First-Person Shooter which utilizes the famous and popular Unreal Engine technology. Complete with campaign which utilizes several game modes also found in its multiplayer.
Unturned is a free to play zombie survival RPG via Steam. Kill zombies. Gather resources. Level up skills. Kill zombies. Craft items and materials. Survive.
XCOM: Enemy Unknown is a turn based tactical strategy game for PC. Players command an elite military organization to defend the world against alien invasion. On top of tactical combat, players manage XCOM's bases and maintain their income through appeasing the world's nations by partaking in missions in their country to keep public hysteria to a minimum.