You can get a copy of SpaceChem from the developer's site. At first startup it needs an activation code which can be found in the ~/Steam/SteamApps/common/SpaceChem/registration.key file. After that the game writes a database with save states and the activation somewhere. Save states from the Steam version are usable in the normal version, at least under Linux.
I couldn't check if the normal game really is DRM-free, but the Linux version sure is and the game is at 85% sale at bundle stars at the moment ;).

I'm running a Mac (OS X Mavericks).

I was able to move the following games out of the Steam folder and play them without the Steam client open/running just by clicking on the moved icon:

The Bard's Tale (2004)
Shattered Haven
Shelter

I was wondering, when you get some time, do you think you could test a couple "triple-A" titles, if you have any? From what I understand, the Mac version of Steam doesn't even have the CEG component, effectively making pretty much any game that relies on it DRM-free on the Mac. I know The Walking Dead, Shadowrun Returns, and Deus Ex: Human Revolution are all DRM-free on Mac, but not Windows, and I'd love to see if bigger stuff works, like Bioshock Infinite or something.

I haven't got any games like that. The only other Steam games I have are Duke Nukem 3D and Shadow Warrior. I tried moving Duke, but it wouldn't open after I moved the files out of the Steam folder. I don't have much technical expertise, so I didn't try anything beyond that.

ETA: I don't know if I tried running dosbox first and then trying to play Duke it might or might not work? I've never tried running dosbox before, outside of auto-running it when I click a game's icon to play.

ETA2: Oh, I do have L4D2 but haven't finished downloading it yet, I have a monthly bandwidth cap so I'm waiting until closer to the end of the month to see if I have room to complete it.

Okay, that's cool. I just thought I'd check. Thanks for the response!

Left 4 Dead would be really interesting, seeing as how it's actually made by Valve. And as far as DOSBOX goes, all those games are DRM-free; it's just the nature of the emulator.

I stumbled yesterday across an interesting site. It's called drmdatabase.net and seems to track all sorts of DRM on various games for Windows. Among those games are many Steam titles so it's like a gold mine. It also provides a little tool to check games for potential DRM (sadly only for Windows).

For anyone who read my report that The Swapper is DRM-free through Steam, the game was just updated, adding the file CSteamworks.dll. This file has to be deleted in order to bypass Steam now, otherwise, the client WILL start, or the program will crash if Steam isn't found.
Oh, I didn't know they had a tool. I knew about the site (although I've seen some misinformation, if I recall correctly), but I'll have to check the tool out.

Linux versions of Avadon - The Black Fortress and Uplink are DRM free on Steam. Just tested myself.

Guys, can someone of you Broken Age backers check if part one of that game, released yesterday Steam exclusive for backers, is DRM'd?

I need to settle a dispute. ;)

Downloading the Linux version in Steam at the moment. I will update my post when it's finished and checked, so please wait ;)

Edit: The Linux version is DRM free, do you need the status of the Windows version, too?

Edit 2: The Windows version does need the Steamworks CEG, it won't start without the Steam launcher.

Edit 3: Do you mean DRM in general? Despite the needed launcher under Windows it's completely clean. I can't say anything about the Mac version, though.

I just checked, and I can happily report that the version of Retrovirus on Steam is completely free of DRM. Furthermore, the "DRM-Free version" has complete Steam integration even if you have never installed the game through Steam. The two versions are, in fact, one and the same in every way and the best of both worlds. This is the way everyone should do it.

I couldn't have said it better myself. There's no reason to lock games to Steam when developers can provide all the benefits of the platform without any of the restrictions.

Also, anyone who had Starbound on Steam, can you check the DRM status of that? I've heard it doesn't require the client.

And on a personal note, I'm interested to see if Nidhogg is DRM-free, too.

I would be interested in Nidhogg, too. It seems like an awesome offline-multiplayer-game to rage with your friends.

I can confirm that you don't need Steam to run the Starbound Launcher and the game executables (regular and OpenGL). They're located in the win32 folder in the Starbound folder.