Daggerfall Unity needs the original data to be functional. It is not standalone. But as it has been said, Daggerfall is officially free in the Elder Scrolls website. It is even added for free to your library with any bethesda purchase in GOG.

Warzone is good even for an old game.
I havent play the others.

Mmmm... I think is more a source port, than a standalone game, I will make a bonus section with other games/mods that are not OPL Standalone Games and will add it.
By the way, I see your list but they already recommended, I'm trying to make a list with personal recommendations of games that are on a playable state and that are free, if you have one, let me know and I will add it.

C-Evo, a Civ II clone. A bit different, unlike Freeciv, somewhere in-between Civ II and III.

Domino Chain, which is essentially an open source version of the Push-Over game with more modes, puzzles, and unique pieces.

Enigma, an open-source remake of Oxyd and its sequels.

My mistake has been corrected :D

Added to the list.
Done, now is on the list.
Done, entry added!

To elaborate a bit more on my previous post :
scorched3d it's a weird artillery game, here's the Wikipedia entry

Lugaru yeah those man-rabbits fighters are Open Source. Not sure if the assets are included with source code, the git version is roughly the same 45MB size as Steam version, so probably yes (not tested).

Aquaria does indeed require the game to be bought in order to play it but the engine is Open Source . The game is nothing short of beautifull, available on GOG and is indeed a great relaxing game.
The OP probably want complete, ready to play and gratis games, so Aquaria doesn't fit 100% there but still Open Source.

Have you tried Scorched 3D? Looks like a worms clone but I can idea from the trailer, I will read the Wikipedia to see if I can get an idea or I will test it during the weekend.
About Aquaria, that game looks amazing, I got it on my wishlist for quite some time, I'm waiting for a good offer, and yeah, the source code is just for modding, but heard is a great metroidvania.
About Lugaru, for what I read just the source code and some assets are free, not the hole game, I haven't download anything yet, so I will test it later, or if you can try it and tell me I will appreciated :)

New entry added.
Entry added, heard is really good, I haven't try it yet but I have Jupiter Hell on my wishlist.

Lugaru is confirmed to work just fine, just downloaded the game and did a couple of missions, may I add that the game is actually cool. Is some sort of 3D man-rabbit fighting game with some weird-ish controls that end up to work ok, after the 5-10 minutes of the tuturial.
Can't go wrong with a 45MB Zip file to get a few hours of enjoyment, fighting the evil bastards to liberate the bunnies!

Don't forget One Must Fall: 2097 - https://www.myabandonware.com/game/one-must-fall-2097-2a7

Alright. HyperRogue: It's a rogue-like set on a hyperbolic plane with interesting geometric potentials; in exchange of complex mechanics you get complex landscapes.