This is a list of games available on Steam that do not use the Steamworks CEG component, making them effectively DRM-free once installed. This means that you can copy the game folder anywhere you want and launch the .exe directly without being online or having Steam running.

1000 Amps
All Zombies Must Die* (run ShippingPC-Bzb2Game.exe)
Alpha Protocol (run Binaries\APGame.exe, NOT APLauncher.exe)
Analogue: A Hate Story
A.R.E.S.: Extinction Agenda (error message can be dismissed)
Atom Zombie Smasher (run atom_net.exe, NOT atomzombiesmasher.exe)
Aztaka
Basement Collection, The (doesn't show unlocked content – see here)
Bastion
BEEP
Ben There, Dan That
Binding of Isaac, The
Blackwell series (achievements disabled)
Blocks That Matter
Bob Came in Pieces
Botanicula
Breath of Death VII
Broken Sword 2
Cogs
Containment
Cortex Command
Crayon Physics Deluxe
Crusader Kings
Crusader Kings 2
Cthulhu Saves the World
Dangerous High School Girls in Trouble!
Dangerous Waters
Data Jammers: FastForward
DCS: Black Shark
DEFCON
Divine Divinity
Dream Machine, The
Dungeon Siege 2 (but not 1 or 3)
Dungeons of Dredmor (mods installed through the Steam Workshop aren't available)
Elven Legacy
Ether Vapor Remaster
Eufloria
Fable: The Lost Chapters
Faerie Solitaire
Fallout 3
Far Cry 2
Frozen Synapse
Galactic Civilizations II: Ultimate Edition
Gemini Rue
Grand Theft Auto
Grand Theft Auto 2
Gratuitous Space Battles
Hacker Evolution Duality
Home
Jamestown
Legend of Grimrock (will launch Steam if it can be found, but runs if it cannot – see here)
Legendary
Lightfish*
Lone Survivor
Lucid
Lunar Flight
Machinarium
Majesty HD (but not Majesty Gold – see here)
Majesty 2 Collection
Mini Ninjas
Misadventures of P.B. Winterbottom, The
?Mount and Blade: Warband (version 1.153 was CEG free, 1.154 isn't; Steam CD key can be used to redeem a DRM-free copy – see here)
Multiwinia
Nimbus
Noitu Love 2: Devolution
NyxQuest: Kindred Spirits
Oddworld: Munch's Oddysee
Oddworld: Stranger's Wrath HD
Painkiller Redemption
Path, The
PixelJunk Eden* (error message can be dismissed)
Polynomial, The
Psychonauts
Puzzle Quest 2
Rayman Origins
Really Big Sky
Rock of Ages*
Saira
Samorost 2
Sega Genesis / Mega Drive Classics
Serious Sam 2 (but neither of the Serious Sam HD Encounters)
Sniper Elite
Snuggle Truck
SOL: Exodus
Sonic the Hedgehog 4 – Episode I (but not Episode II)
Space Rangers
Super Crate Box
Super Laser Racer
Super Meat Boy
Tales of Monkey Island – Episode 1 (but not the remaining episodes)
Thief: The Dark Project
Thief II: The Metal Age
Time, Gentlemen, Please!
TRAUMA
Trapped Dead
Unstoppable Gorg
Vertex Dispenser
Victoria II
Virus Named TOM, A
VVVVVV
Windosill
Wizorb
Worms Pinball

List of DOSBox games on Steam
All DOSBox games on Steam, including those not on this list, are DRM-free.
Commander Keen series
Deathkings of the Dark Citadel
Doom, Doom II, Master Levels and Final Doom
Heretic
Hexen
King's Quest and Space Quest Collections (the launcher uses CEG, the individual games are DOSBoxed)
Star Wars: Dark Forces
X-COM: UFO Defense, Terror from the Deep and Apocalypse
Wolfenstein 3D and Spear of Destiny
Worms

A question mark means the game is rumoured to be DRM-free, but confirmation is needed.
Asterisks mark games that are Steam exclusives, i.e. only available from Steam; buying them elsewhere only nets you a Steam key.
Note that games with any third party DRM are disqualified from this list, even if they do not actually use CEG (such as C&C4 or Batman:AA).

Big thanks to everyone who contributed to this list, particularly Skunk who provided most of the early data, iainmet and amok, our correspondent extraordinaire. This is a community-built list and very much a work in progress, so all of your input is most welcome!

If anyone wants to help by checking Steam games, first look at this pastebin of games we have already tested so you don't have to waste your time.

FAQ:

Q: But Steam is DRM!
A: That's not a question. And it isn't technically true. Steam's DRM solution (called "CEG" by Valve) is just one of many Steamworks components game developers may use, alongside achievements, cloud saving, the workshop, matchmaking network code etc.; just like with all the others, implementing it is not required to distribute a game on Steam. A game can use the other Steamworks features and still remain DRM-free.

Q: How do you determine if a game is DRM-free on Steam?
A: Simple: install the game and launch it once (this step is important, because this finalises the installation). Shut down Steam and temporarily move all the files in the root Steam folder somewhere else. Locate game folder, locate the .exe, run it. Either you get an error message (or it does nothing at all), or the game just runs. If it just runs, it is DRM-free.

Q: Game X has a source port which makes it effectively DRM-free.
A: True, but that isn't quite what this list is about. Obviously if you don't use the game executable at all, you are bypassing the CEG ("Custom Executable Generation") component completely. The idea is to determine whether the CEG component is actually present and active, which is something not advertised by Valve at all.

So do those games still require the steam client?

DCS Black Shark is also DRM free. I'd also hazard a guess Serious Sam The Random Encounter and Super Crate Box are too.

I believe Crusader Kings 2 is also DRM free in this regard.

"This means that you can copy the game folder anywhere you want and launch the .exe directly without being online or having Steam running. "

Second sentence in the OP

Edit: and +1 for this thread. Though I won't be contributing much.

only to download. You can play them after without the client.

Yeah, but is it that offline mode steam option, or completely GOG style fully offline?

To add to your list :

Divine Divinity
Thief : The Dark Project
Thief II : The Metal Age

You can literally just run the exe file once installed and it will start the game without starting the Steam client.

Oh, I'm pretty sure that Crusader Kings 2 is actually DRM-free. It definitely launched without going through Steam, though Steam was running (it usually is).

I also remember being able to run Fallout 3 without the steam client. Does this still apply? might check FNV later if it is still possible.

Useful information, thank you for this. I'd assumed Steamworks meant DRM every time I saw it - good to know that's not how it works, albeit very irritating that they don't make it clear whether a game uses that component or not before you purchase it.

I don't have Fallout 3 on Steam, but New Vegas has CEG.

"without [...] having Steam running", it says.

Steam's offline mode still requires running Steam, and so does not fill the above criteria.

Afaik, they can't make every game use the Steamworks DRM. Developers are forced to use some part of the Steamworks API usually (well, most of them. DCS Black Shark doesn't even have the API's DLL in its folder) but it's generally only for a few features, and afaik, it's entirely up to the developers just how much DRM is included.

I actually don't think a full Steamworks interface exists for Game Maker, and yet Steam is selling Game Maker games. In fact, Super Crate Box is one of them (albeit that one is free).