- SuperTuxKart
I really like this game because it's basically just FOSS Mario Kart. It has a lot of gamemodes, an (admittedly simple) kart stat system, many tracks and a lot of mods.
- OpenTTD
In OpenTTD, you transport cargo and people to different places. You can transport them using trains, buses, aeroplanes (airplanes if your American), boats and trams if you add a NewGRF (a mod). It is very addictive and it is quite easy to get into (except the signals).
- Pingus
It's FOSS Lemmings with penguins.
- Extreme Tux Racer
You play as Tux (or someone else) and slide yourself through terrain while you collect fish. It is fun, however I've found that it can get a bit repetitive. You can design levels, however it's a bit hard to do.
- SuperTux
It's 2D Mario with a penguin. BEST GAME EVER.
If you aren't already use shift click to que up multiple commands for units (this works for any command in the game). Once you get used to relying on it everything becomes way more manageable!
https://www.beyondallreason.info/commands/move
Also set your factories to "repeat" and set up defensive patrol routes with your factories so you can sit back a bit and not have to micro everything as hard.
Also the community is friendly don't be afraid to ask for help in getting better!
Space-click is such a nice feature. I stopped wishing I could edit queues once I found it.