I was wondering what other digital retailers are completely DRM in the same vein as GOG (as in the ability to back up all installers without limits, reinstall without activation unlimited times, etc.). Even if it is just limited to certain titles. The only other one I've heard of is Amazon, but I have trouble finding which ones specifically have no DRM.

Some games on GamersGate are DRM free. They generally have that information on the product pages. (But you need to back up the installer files during the installation as they are automatically deleted afterwards.)

GamersGate has a decent number of DRM free games, but not everything is DRM free. They've got labels for the DRM, though unfortunately, many are unlabeled, and others just list "Other DRM" without giving any other information about it.

I've heard people mention that their DRM-free advertisement is not really DRM free, is this why?

The launcher requires internet connection to "decrypt" the game data into an executable installer during the installation process. If you copy that decrypted installer, you can back it up and later on you can install without internet connection.

So, DRM-free, but with an effort :-)

It's a tricky one. The DRM-free games really are DRM-free, but their downloader likes to encrypt the installer if you choose to save the installation files after installing. There are very easy ways around this though (ending the downloader process using the task manager while the installer is still running is my preferred method).

EDIT: Ninja'd!

Indievania is another, obviously focusing on indie titles.

They do have some games that aren't DRM-free but they carry a warning.

Yes, that's correct. The games marked as DRM free by GamersGate really are DRM free, but you need to back up the installer files manually because they are deleted by the downloader once the installation is finished. As others have said, you can do this by copying the installer (usually called setup.exe) or by shutting down the downloader during the installation (before it has the chance to delete the installer).

Another thing worth noting is that games marked as "other DRM" are sometimes DRM free too. GamersGate regards serial numbers as DRM, so sometimes games listed under "other DRM" only require you to enter a serial number offline. This is not true for every game listed as "other DRM", however.

GamersFront (Shrapnel Games' store - all DRM free)
ShowMeTheGames - list of indie games, mostly DRM-free (Evochron Mercenary is the only exception I've seen)
- a "pay what you please bundle" (like [url=http://www.humblebundle.com/]HumbleBundle without the charity) with mostly DRM-free titles (they have had a couple of games that were Steam-only - you can ask for them to be removed from your bundle though, to ensure the developers receive no benefit).

Do you mean services which offer only DRM-free titles, or some DRM-free titles?

If the former, I know only of DotEmu (maybe some indie sites too).

If the latter, pretty much everyone besides OnLive offers some DRM-free titles, even Steam.

As I found out recently Desura is offering more and more DRM-free games, but not all of them are without DRM at the moment.
Also Humble Store from Humble Bundle guys is committed to providing DRM-free versions of the games. That does not mean you get every game DRM-free with them, but they promise to provide DRM-free versions for those games that do not have them currently in the future. (e.g. for Trine 2 that is the case).

Are you sure? I thought every title bought on Steam requires the Steam client to authenticate. I'm looking for either all or a selection of titles.

I bought Vessel through this, I think it's a great idea. I have a humble key e-mail that links all my bundle purchases and that through my e-mail address, though I don't think they have a dedicated storefront site like GOG yet.