Mindustry, a game that mixes in Factorio with Tower Defence and then goes wild from there is a fantastic example of how good a free and open source game can be.
Currently available as a tech demo with the developer needing feedback, Ova Magica looks like a promising start of a new casual sim blending lots of different gameplay features.
Open-world space action sim EVERSPACE 2 from ROCKFISH Games has shown off plenty of new footage across multiple events recently and it looks seriously shiny.
Untrusted, an upcoming online indie game of hacking and social deduction inspired by the likes of Werewolf/Mafia genre and others similar is running a free stress-test soon.
Snowtopia, currently in development with a free version available while it's early on continues to be a promising new building sim that has you build a ski resort.
Currently in development with it classed as being in an "alpha state", Verdant Village seems like a sweet and promising high fantasy alternative to Stardew Valley with some new themes.
Untrusted is similar in idea to other social deduction games like Mafia/Werewolf (amongst plenty of others), only this time it's a bit more nerdy and technical with the hacking idea.
One of my most anticipated releases to come from a crowdfunding campaign, Space Haven continues to get some really fun features and another huge Alpha release recently went up.
Space Station 13 is something of a classic, released way back in 2003 on the BYOND service. It's being remade with Unity to have a more modern experience, as well as being cross-platform.
Last August, the developer responsible for the excellent mod Brutal Doom, revealed after a month of absence that he was working on a new project called VietDoom, which takes place during Vietnam War.
The Fertile Crescent is an upcoming in-development indie RTS that feels like a retro Age of Empires and it's really quite good. A new update is out (and it's still free) fixing up the UI for different resolutions.
With months of development behind them, Godot Engine 3.2 hits the first Alpha release with a ton of new and improved features for this FOSS game engine.
Move over Cube World, there's a new open source voxel RPG in town under development called Veloren and it supports Linux too so there's a bonus point for you.
Somewhat inspired by the satire of Starship Troopers, Second Earth is the current free prototype base building strategy game from Free Lives (developer of Broforce).
Up for playing another building and management tycoon game? Snowtopia: Ski Resort Tycoon is one we talked about briefly back in January and it seems to be progressing along nicely.