Right. I missed it. :/

Book of Unwritten Tales

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"Finding Teddy"
This nice little indie adventure is DRM-free after the steam download. Tested on Win7/64.

I bought this on Indie Game Stand (it's on sale atm) including the officiel DRM-free version. I could not get the normal version to run at all and after having to use the steam version instead (and of course testing it for client requirement) it works. I found out that the versions are completely different and already contacted Indie Game Stand and the devs about it. So if you should have problem with the official DRM-free version of the game you should try the Steam version until they have fixed this.

Okay, we're all updated again!

So, we're confirming that Morrowind runs without Steam, correct?

I was able to find that another game is DRM-free.
Pirates of Black Cove Gold

The game was downloaded through a Windows XP virtual machine containing Steam, and ran on the host machine running Windows 10 without Steam installed fine. I was able to take control of the ship, sail a bit, and shoot the two ships that appeared with cannons.

EDIT:
In addition, I found out that Woodle Tree Adventures is also DRM-free from Steam using the same procedure as outline above. It even has the soundtrack in MP3 form, ready to use by your favorite music player!

Who knew that when I download this game that I wouldn't even have to try and figure out which files to remove to get it running without Steam? I really remember it asking for Steam back then.

EDIT 2:
Just got word that Transistor runs DRM-free on Linux via Steam.

SUBSEQUENT EDITS:
*Verified Space Channel 5 Part 2 through Steam and ran it on my host machine; works dandy. What I remember however, is that it will RUN Steam if it exists, but will not bother if it doesn't and play anyway. Downloaded through a XP VM and played through a Windows 10 host machine up to the main menu.

*Robowars which is given away by IndieGala is also DRM-free, same Windows and virtual machine method

*Barrier X is also DRM-free, Windows+virtual machine

*Crazy Taxi is the same case as Space Channel 5.

*RPG Maker games, depending on their nature, can be replaced with generic RPG Maker executables and run safely and without Steam. For instance, Memories of a Vagabond can be made DRM-free by finding a generic RGSS executable, such as the one I provide here. In order to do that, you must go to your game's directory and enter the properties of the executable and read the description. If it says "RGSS Player," you can replace it with the exec I linked above. If it says "RGSS2 Player," you should replace it with this executable, and if it says "RGSS3 Player," you should replace it with this executable. The games might run and might not run, so you should report those that do work that way.

*Pixel Puzzles Japan. Windows + virtual machine test.

*Red Faction can be made DRM-free through the official patch provided here. Simply copy and paste the downloaded file to your Red Faction directory. I couldn't get Pure Faction to run on Windows 10 though.

*Super Sanctum TD. Windows + virtual machine test.

*SEGA Bass Fishing, same as Crazy Taxi and Space Channel 5.

ENKI

The Last NightMary - A Lenda do Cabeça de Cuia

Waste Walkers

All three can be played without steam client running.
Tested on Win7/64

X-Blades joins the fray.

Again, downloaded through the Windows XP virtual machine to my Windows 10 host, it runs neatly, without Steam.

Here is another bunch:

Nicolas Eymerich - The Inquisitor - Book 1: The Plague (Book 2 DOES need Steam)
Return to Mysterious Island 2
Sinking Island

Tested on Win7/64

Alwa's Awakening

http://store.steampowered.com/app/549260

Tested on Win10Pro 64bits

Shoot 1Up on Windows 7, run the exe from your directory

Iron Commando

http://store.steampowered.com/app/418640/?l=dutch

Win10 Pro 64 bits

Quake 4 works like a charm without Steam on Windows 7 64 bit.

great list, thank you everyone for updating it :D

Woo, okay, we're updated again!