Besides the previously reported free weekends that are now underway for PAYDAY 2, American Truck Simulator, The Elder Scrolls Online, and Dog Duty, a few more games can be played for free on Steam over the next couple of days (thanks Steam Database via Ant. Here's a rundown:

Just Cause 4 Reloaded (Windows)
Jump into an action-packed open-world sandbox experience and cause chaos with a wide selection of weaponry, vehicles and gear. Strap into your wingsuit, equip your fully customizable grappling hook, and get ready to bring the thunder!

Cities: Skylines (Windows, macOS, Linux)
Cities: Skylines is a modern take on the classic city simulation. The game introduces new game play elements to realize the thrill and hardships of creating and maintaining a real city whilst expanding on some well-established tropes of the city building experience.

Invisigun Reloaded (Windows, macOS, Linux)
Risk versus reward guides your journey through the full-length single player campaign and local/online multiplayer. Vigilance is key as environmental tells such as footprints and other disturbances will point out careless cadets. A massive amount of lovingly-made environments, maps, modes, and abilities – all obsessively balanced – combine for high replayability and a deep path to mastery.

The Detail (Windows, macOS, Linux)
Controlling the actions of a bitter veteran detective investigating a brutal gangland murder and a reformed criminal trying to protect his family, the player is forced to make tough choices and deal with the resulting consequences of those moral dilemmas. The gameplay focuses on branching narrative mixed with interactive comics, and is completed by a professional audio track.

Headsnatchers (Windows)
Hold onto your head with Headsnatchers, a stupidly hectic up-to-4-player local and online multiplayer party-game where you snatch your friends’ heads to come out victorious. Because, let’s face it, you’ll do anything to win.

GOLD EXPRESS (Windows)
In the future when human science and technology are in conflict with the ethics,
what will happen between the machine elites with advanced AI and the transformed humans in the project “Gold Express”? All of these depend on your decision, player!