No personal experience, but according to this list, Gone Home is launcher-free.

Excellent. Good to know.
Thanks! :D

There were recently some Linux ports of AAA games on Steam and I thought I'd share what I found out:

XCOM: Enemy Unknown - still won't work under Linux without Steam. Though it's still cool that there’s now a native port.
System Shock 2 - The "port" is a wine-crossover-bottle-wrapper of the windows game. It runs without CEG and Steam, but I have issues with the resolution, so I can't play it at the moment (not even inside steam). Maybe someone else can confirm this?

I don't know if this really counts, but the Heroes Rise saga uses CEG, but the game is a HTML code game, so you can use (nearly) any browser you want to play the game by simply opening the .html files under Linux. As I see only the build-in browser to play the .html files uses CEG.

Games that use 3rd party DRM are automatically excluded.

Proper test requires to copy the installation folder to another PC that never had steam installed. If the game plays, then it is truly DRM-Free. If it doesn't, the registry entries are subsequently checked. If adding said entries to the non-steam PC allows the game to run, the game is still DRM-Free.
Unfortunately, due to the usual lack of second non-steam PC, running the exe without steam is the usual check. Registry entries can be added much easier, especially since they are found in the game's steam folder.

I'm now home so can Check actual Examples
Fallout 3 is listed but requires a CD key hence has DRM.
Dragon Age Origins Ultimate Edition is not in OP but on page 2 (Not sure if OP is updated, as far as I got)
Copying files and regisrtry entries would also copy such CD key data, which is why the more intrusive methods of DRM such as Disc in Drive and worse are used, as entered keys are so easily copied.

So even the other PC method doesn't prove no DRM, it may be easily broken DRM and pretty useless DRM, but thats never stopped it being used.

Only when it's known for a fact that the CD key is absent by verifiable removal or a download is unactivated can it truly be said to be DRM free.

Now don't get me wrong I'm not saying this thread is pointless, as a CD key is meant to be a first use enter gate and without the more obnoxious DRM it's normally used with, combined with lack of Steam DRM, it's the next best thing to DRM free we can get.
However ithe key must be entered and stored on your PC somewhere in the files or registry, copying these copies the key.

The two games I mention are ones I have that I know need CD Keys, I don't have most of the listed games so likely others exist.

The two PC method isn't really required, simply renaming the Steam Folder while testing should produce valid results. If OP is being updated a short note about CD Key games, along with pointing out the affected games, should be enough. Also it occurs to me that some that need registry info may need CD Key data from registry.

CD-Key by itself is not a DRM. An online check makes said CD-Key DRM. See Diablo 2 for an example, since the offline CD-Key can be any key you want, it is only required to install.
So no, if a second, offline computer that never had steam installed can run a game, that game is DRM-Free.

Added Summoner and UFO Afterlight.

Papers, Please, on Linux at least, is DRM free.

Kerbal Space Program is also DRM free on Linux.

You should read the posts more carefully:
A CD key by itself is not DRM. DRM requires that someone else controls your ability to copy/install/use your game. Generally, this implies that connecting to a server is required at some point. Conversely, some well-known DRM-schemes can be considered DRM-free, if they're implemented loosely enough, e.g. GfWL with offline profile.

This is probably silly to ask, considering I'm sure there are tons of things Valve receives requests about, but is there any way to get them to list the usage of their, let's call it first-party, DRM? Seems like the least they could do with their crazy lockdown on the market.

CD key is DRM (Digital Rights Management) though by todays standards it's ineffective DRM and very mild DRM but DRM none the less.
It is a digital check to mange the right of the game owner to play the game.
No key = No play whether you paid for the game or not, that is DRM. There is no disputing it, it's a basic fact.

GOG uses no CD Keys for single player games, though some multiplayer components of GOG games come with a CD Key, it is the Multiplayer Server that requires it, not GOG.

You are wrong to call such games DRM free because the key is undisputably DRM, the first DRM ever used in fact, long before the name DRM was invented.

The fact it's always been "Useless DRM" is irrelevent, all DRM is useless at stopping piracy, the supposed target. Only honest users are stopped by DRM. We all know this. even Ubisoft admit it now, but they still don't really understand what it means
Ubisoft: DRM Can't Stop Piracy - GameSpot So "Online Services" that paying players may not want is what DRM Free means to them, it just means "Online DRM" to me."
They just don't get what Custermer Service means and never will, Ubisoft and EA are the worst Game publishers there is in my view, they care only about profit, if getting more means exp[loiting paying customers, it's done, over and over, again and again. They pretend to listen and mend their ways. Then do the same crap, over and over, again and again. They cause more piracy by these actions than any DRM has ever stopped.

Preaching to the converted I know, but reading that article raised my hopes and then immediatly dashed them in one sentence.

Again, depends on the game and how the CD-Key is used.

Diablo 2 requires a key to install. Once you installed it, you can copy the installation folder elsewhere, then change the key to "GOG RULEZZZ", and the game will still play while offline.
Attempting to connect to Battle.net will display a "The CD-Key you are using is not for this game", and prevent you from playing online. But offline play is just fine.

Diablo 2 requires a key to install, So No key = No Play.
What's done with files after is irrelevant, the key was an install key, it's already been used and is still DRM stopping you installing a paid for game, if the key is lost.
In fact as you've said yourself it's a dual purpose DRM for Diablo 2.
Install and online Access.
Obviously Offline play requires, no recurring, key entry (few games do) but even so, lose the key and you can't install or play online.

Exact mechanics of any specific key don't matter. If a Key must be used at any point in the process, that key is DRM.

By Definition DRM Free means you only require the install program, no checks of ownership at all, this is how GOG's DRM Free system Works, Online components may require proof of ownership. Usually with a key, that's the online part not GOG.
No game I've bought from GOG requires any proof to access the Game at all.

Arguing that, when the proof is given, somehow, makes a difference to whether a CD Key is DRM or not is false Logic.
The fact a CD Key is required at all, makes it DRM by definition, not when you must use it..

DRM free means, all I need is the game installer, with no checks for anything else at all, ever, none.
No Install limits or any such nonsense. Just the game and only the game from the moment of purchace onwards.
Anything else is DRM.
The relative aggravation, a particular method of DRM gives, makes no difference at all to the fact that it is DRM.
The only thing it does, is determine how annoying, to a legitimate user, that method of DRM is.

Fun fact. Downloadable versions of Diablo 2 don't ask for a key when installing. They do use a key, but for online play.
And there are ways to install without a CD-Key, you just have to go look for them.