- GameBillet sells some DRM Free games.

But make sure to save the files when you buy the games, I once bought the Garfield Kart there (DRM Free) and after a few months the files were no longer available for download. I went to check and they had replaced the DRM Free version on the store for the Steam key version, along with that they probably decided that people who bought the DRM Free game wouldn't need their files anymore (I don't know if that is what happened but it's the only explanation I can think of).

Fortunately I had the files backed up on my computer.

- Gamesplanet sells some GOG keys.

Warning message added, thank you!

ADDED, thank you!

I had earlier submitted Matrix Games which is still listed in OP. However, now reading this topic below, I may have been mistaken. Perhaps a warning or other such disclaimer is appropriate, as if at least some games are DRM-free on there it is worth noting (especially considering the physical shipment option, which is just awesome to see being offered).

https://www.gog.com/forum/general/do_the_slitherinematrix_games_stores_provide_drmfree_downloads_of_their_games

Several GOG games also use the "cd-key", so I guess it is good enough.
I moved Matrix Games to the "Wait for further confirmation" section anyway.

Matrix has always been using CD-keys; I've been on that store for quite a few years and I have a reasonable amount of games there, they all have these keys.
but you can still backup the installed game files, so you'll never need the keys again. but just backing up the installation files and losing access to your account could be a problem, yes.

I'm a little confused at how you say this.

Does it work like this:
Every time you re-install the game, you need to enter the key? Does activation have to be done with internet/account being signed in for each re-installation?

it's like the Securom crap or CD-keys in general: if you want to install it, you have to have the key. that's always the case, no matter how many times you've previously installed the game on your computer.
but you could also backup the game AFTER you've installed it, so you bypass the installing procedure - that way you'll never need the key again.
most people make a backup of the downloaded files and not the fully installed game; but tbh the Matrix games tend to be of comparatively small size.

The Matrix System is the classic and nnocent method of a CD Key and an installer. With the option of purchasing a phisical copy with your dvd case, your cd, your CD Key and your printed color manual. This version is More expensive, of course.

I never had any problem with this method. can it be considered DRM ? I believe it can. But nothing dramatic.

There are much worse methods out there like the Battlefront store Method. You have your installers and your CD Key but you are limited in the installations´ number you can have with that CD key. Once you spend the CD Key you need to contact service to reset it and be capable of installing the game again. A hassle. too draconian and unnecesary.

I am not familiar with how Securom works but I do understand CD-keys, so I assume there is nothing needed in terms of registry etc that needs to be backed up? In other words, if you have the folder where you installed the game, you can then move it and play it on a brand new offline computer with no issue? That is at least how I am understanding this. Please correct me if I'm wrong, and thank you for taking the time to explain already.
Thank you for your explanation as well. I do technically consider it a DRM by definition, but (this will sound paradoxical) it is not the type of DRM I am primarily worried about since if is easy enough to backup and have on hand. In fact I would much rather gaming return to this style system (if it must have any DRM, though I still think it shouldn't) than what we have now.

Well, it's copy protection, but from a time where the term DRM didn't exist yet.
That only became popular once media (video/music/games) became downloadable and we only got licences, but no physical media. Also the first keys could be activated offline. The online activation of games only became popular later.
(edit: Also some video media included a digital copy that had to be activated online)

Now terms like "Online-DRM" do exist, but they are made up (like software pirating), since the early days the term "DRM" was used, it was always connected to online verification (at least one time, even if after that the licence became permanent). I would say the question if something is to be considered DRM or not depends on if something you already have on your hard disc or CD has to be activated online or not in order to play.

Thanks for all the discussion and clarification about Matrix Games.
I moved it back to main section with the caution:

- Copy-protected by "cd-key" but you can backup it.

Feel free to suggest better wording or provide further information. :-)

I understand "CD-key" protection as the behavior that I install the game, it asks me for a cd-key and done. No account or Internet connection is required.

Online verified cd-key is the next lvl of drm.

How you can consider this as drm-free?
In this way, it can be said that most games on client-based platforms are drm-free. If I understand correctly in this case you have a semi-client implemented in each game installation file.

If you call account details + code + online verification as none-drm, I don't know what it might be.

In a few words:
the description is too short and misleading.

If I'm right, it should be:
"- Copy-protected by online "cd-key" verification (account required), you MUST BACK UP the game folder AFTER a verified INSTALLATION to make it drm-free"

Capital letters are intentional to emphasize the uniqueness of the situation.

I could have misunderstood something due to my lack of knowledge of English.

From what @springtoiffel and @Gudadantza described, I "thought" Matrix games uses a offline cd-key, which is less harmful than online verification.

Before they or others provide further information (offline key vs online verification), I put the temporary warning in #1 for now:

- Copy-protected by "cd-key" but you can backup game files after you installed it.

But what does mean "you can backup game files after you installed it"? What's wrong with backup your game installer and copy your CD key code? In the near past you owned your Game CD and the CD key code printed on it. Not too different. Not different at all.

If I am not mistaken the CD Key use to be added to the registry.

If someone likes to experiment, please try to activate the key of a recently bought but still not activated game while being offline. If that works, then I don't see any problem at all.
The only problem would be a necessary online verification.