Muri is DRM-free on Steam (and not a DOS game, despite the looks).

New DRM-free:

Momodora III
Ring Runner: Flight of the Sages

Tropico 5 Linux
Unsure about the Windows and Mac versions, but I would assume it would be the same.

It checks for a running instance of the Steam API, but skips if the API cannot be found, the game continues to load.

(full disclosure: I'm one of the developer and I'm outrageously posting it here)

BOMB is the aerial-combat game you get when you mix Crimson Skies and Porco Rosso.
store.steampowered.com/app/301500

No DRM whatsoever, you can copy the game's folder and put it right away on your USB stick or BitTorrent!

One limitation though: no server browser outside of Steam, just direct IP connections.

edit: sorry, can't post URLs at the moment...

Okay, I've updated the list with all the new comments here. Thanks again, guys! I also added Sonic Adventure DX, which needs Steam.exe to be renamed in order for it to not start the client.

Holy crap, I love people like you. I need to buy your game just to support you guys for being so cool! Is it both the Windows and Linux versions that are DRM-free? Thanks again for the info!

I think you can add Fallout, Fallout 2 and Fallout Tactics to the list.

"The List" doesn't appear to have been updated for nearly 3 years.

Ignore the first post, that user hasn't been active in a very long time. The list that is still getting updated can be found here:

http://steam.wikia.com/wiki/List_of_DRM-free_games

We should actually probably start a new thread and try to get this one archived -- pedrovay2003 has probably been the most active about updating the wiki so I wonder if he might be up for it?

Would be a good idea. This thread is fairly ancient.

Thank you for everything you're doing! I think you forgot to mention that in order for Tropico 5 to run without Steam you need to rename/delete the steam_api.dll or steam.exe. At least that's how I understood it.

Also Interstellaria (tested on Windows 7) can be added to the list. Although it has a steam_api.dll in the root directory it runs without Steam. Keep in mind: When deleting this file the game DOESN'T even start! So it appears that there's some kind of check if the dll is present, but there's no need for the Steam client to be running for the game to work.

Oh, I see. A new thread is a good idea. I couldn't figure out what you were talking about.

I wouldn't mind starting a new thread, sure. Give me a bit to set it up, and I'll post it.

Yes both, and the mac version which will be available when we're out from early access, in about a week.
Unfortunately we've been rejected by GOG.com :(

They say that the genre is not popular enough.
It seems like GOG is very picky now, too bad, being present on GOG would have spoken for our game's quality.
But my personal disappointment aside, maybe it's a good idea to distinguish themselves from the other online stores.