These are the FLOSS games that stand out, list your own favourites or most-play games.
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I would rate all of these, as worth a try:
Shattered Pixel Dungeon
Cube2:Sauerbraten
SGT-Puzzles
Andor's Trail
AssaultCube
Minetest
Neverball/Neverputt
PowderToy
0ad
Fillets-ng
Anuto TD
Xmoto/Bloboats
Flightgear
Kobo Deluxe
Enigma (oxyd)
LiquidWar5
H-Craft Championship
Numpty Physics
Wesnoth
The Dark mod
Have completed SuperTuxKart, BlobWars 1&2, Flare, Frozen Bubble, Hex-a-Hop, Holotz's Castle, SearchAndRescue II, Alex the Alligator, Project:Starfighter, Stormbaan Coureur, Trigger, all are fun.
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Can find details about most of the above games here:
https://libregamewiki.org/List_of_games
FLOSS gaming is excellent, thanks to all these devs and asset creators.
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BONUS TOMT:
I'm looking for the name of a FLOSS Quake1-mod, puzzle game, that was about placing gravity points, to curve a stream of particles around the level, and eventually into the goal target. (May have used irrlicht)
If anyone knows the name of this one, please let me know, it is my white-whale of games.
Thrive: the cell-stage in Spore but more in-depth.
You float around looking for various compounds for energy and to grow. After growing you can add parts to the cell or modify it's membrane type or the behavior of AI-controlled copies (you lose if there are no others of your species when you die). You can also migrate to a new environment with different resources (including sunlight). The game ends when you live long enough or complete evolutionary objectives for which the next stage is a work in progress. It can often be easy or difficult to survive, depending on your choices.
Protip: you can modify the cell to be faster at the expense of less HP - would recommend.
Thrive looks like an excellent evolution game.
I haven't tried it yet, but did watch the retrospective video from one of the core developers who joined the Dev team as a teen. Can't find the link for that video, can anyone post it?
Good gameplay tips.
-= Addition =-
Here is the link for the Thrive overview video, mentioned above(warning 46min long): link