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

Fish Fillets - http://fillets.sourceforge.net/

Can you provide a link to developers page where they give that game away for free? The site you linked is not legal.

MyAbandonware is a well-known site, and I'm pretty sure game preservation is quite legal (not to mention really important)...
Despite that, yes it is in a grey area, so here are the sources -

Wikipedia - [url=https://en.wikipedia.org/wiki/One_Must_Fall:_2097]https://en.wikipedia.org/wiki/One_Must_Fall:_2097[/url]
Says that it was declared freeware by the developers in 1999. I think freeware is permitted to be shared on websites.

Developer's website - https://web.archive.org/web/19990427155017/http://www.omf.com
No use actually. It was archived in 1998-1999, after OMF: Battlegrounds' announcement, so it says nothing. Still thought I should link it.

Archive.org - https://archive.org/details/msdos_One_Must_Fall_2097_1994
Found this as well.

In terms of this particular game, if it’s given away for free by the IP owner then that is fine. MyAbandonware is not however legal. It does not matter how well known something is (ever heard of a certain bay?). And no, there is no legal basis for game preservation. Just saying, I don’t agree with it, but that is the way things are. IP holders own the IP and are the only ones allowed to give it away, sell it, etc. Abandonware does not exist legally, archive.org is on shaky ground at best as well, though I doubt most would chase for such old content.

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

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 ;)]

OMF was made freeware, yes, but it's not open source. OpenOMF is, but to my knowledge it's not quite ready.
Abandonware however is not a legal concept and sharing "abandonware" games is, in fact, very much illegal. There is nothing grey about it legally.

There's an open source version of the famous troll platformer Syobon Action called "Open Syobon".

Worth noting that the assets, in particular the audio, aren't completely legal (though the first stage music, taken from the legendary terrible game "Action 52", is in a strange copyright limbo, so while getting permission to use it is likely impossible, it's similarly unlikely that you'd get sued, but a company wouldn't risk it). On the other hand, the game as a whole could be argued to be fair use as it does appare to be a parody of Super Mario Bros..

Edit: IANAL

I put a post up in the Push-Over forum on importing the levels from the original Push-Over into Domino-Chain and updating some textures to the classic ones.

It's a free open source game or is a proprietary game open source code?
Great, thanks! Entry added.

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".

Star Ruler 2 is also open source despite being sold on steam and GOG.


https://github.com/BlindMindStudios/StarRuler2-Source

Lemme elaborate on this 'cause i was reading replies and took special notice of games that are on sale on gog. Indeed you are correct, but one of the comments i read on the topic is interesting: apparently the engine itself is also called Aquaria. So technically the devs never lied, despite all the butthurt seen in the comments, however you a right in that this is most likely not what the OP wants.

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.