thraxman: Was thinking of creating a topic like this myself, well don't have to keep it updated or curate it now thanks, here's some good games your missing:

Sonic Robo Blast 2 - a recreation of the cancelled sonic Sega Saturn game using GZDOOM
https://www.srb2.org/

Super Mario PC Port - a port of Mario 64 to PC with options to upscale or modify graphics and textures (does require you to provide a rom, but not like you won't find one if you want to).
http://www.sm64pc.info/

Total Chaos - Another GZDOOM TC set in a abandoned haunted city resembling Hashima. Creator has tweaked GZDOOM though to look like Unreal 3.
https://www.moddb.com/mods/total-chaos

Duke Nukem: Alien Armageddon - Best Duke 3D mod you will find, adds new campaigns, weapons and characters (like the original bombshell) the play along side the originals and complement the perfectly.
https://www.moddb.com/mods/duke-nukem-alien-armageddon

Renegade X - remake of C&C Renegade on Unreal 3.
https://ren-x.com/

Ashes Afterglow - A GZDOOM mod set in a Mad Max like post apoc world, still being developed first episode is complete and a demo of the second is available.
https://forum.zdoom.org/viewtopic.php?t=47573

The Ur-Quan Masters - Yer its creators ain't done themselves any favours on the PR front but this still exists and it isn't that bad.
http://sc2.sourceforge.net/downloads.php

viperfdl: Rather a stand alone mod than an open source game but STALKER: Anomaly is worth playing while waiting for STALKER 2.

Splatsch: Unciv is an open source pixel remake of Civ V. It's really enjoyable :) (and light)

Minetest a nice open source Minecraft-like clone !

Simutrans : I heard lot of good things about this one ! An OpenTTD-like game !

FreeOrion An open source remake of Master of Orion !

[by the way, it's Battle for Wesnoth, not Battle for Wesnorth ;)]

ConanTheBald: FreeCol

"FreeCol is a turn-based strategy game based on the old game Colonization, and similar to Civilization."

Unknown Horizons

"A 2D realtime strategy simulation with an emphasis on economy and city building."

Basically it's Anno style "City Builder".

sanscript: GL Tron - http://www.gltron.org/

Red Eclipse (MP FPS Arena Shooter) - https://www.redeclipse.net/

matterbandit: Thx for creating this thread, KetobaK! +1 and added to my favourites. ;)

Many apps and software that I rely on are open source, but it never occurred to me to search for open source games! A couple have already caught my eye: Minetest, Blade of Agony, Xonotic, Witch Blast, Hedgewars, Teeworlds, A Dark Room, STALKER: Anomaly (ok, not open source, but still caught my eye).

Question: is it safe for me to assume that open source games are drm-free by default?

Lone_Scout: RuneSword 2

Runesword II (RSII) is a CRPG designed to appeal to the "pen & paper" role-player. RSII features turn-based, strategic combat, 50+ skills, 12+ races, and several full-length adventures to play set in the world of Eternia; not to mention adventures to play in other world settings. What really makes RuneSword stand out is a flexible, easy-to-use engine for creating your own adventures and even your own worlds. But, ease-of-use does not mean wimpy or limited; RuneSword has a robust scripting engine that allows you to do just aout anything you can imagine.

nightcraw1er.488: Clue is in the “open source”, yes it would be drm free. Also, there is no point adding drm (actually I could stop there) as these are free.