This.

External HDD.

Currently only my "general downloads and other unsorted files" HDD. Whenever I decide to spend the money needed to build a relatively decent network storage solution (ie. conforming to the specs I already drew up), I'll move them there (together with everything else on said drive, and a few others).

External Hard-drive!

External HDD, mostly so I don't have to download the game I decide to install. I download them when I buy them or when they are updated, then move them to the external HDD.

DVD? Many GOG games already exceed the size of DVDs, I think. I'm actually in the process of doing the opposite, moving the contents of some of my old CD-R and DVD-R discs into external hard drives.

Those GOG games which I back up, I back up to a 2 terabyte SATA drive(s) I bought the other day. They are connected to my laptop through an USB case into which I connect the SATA drives.

The most critical data I back up to two different hard drives, in case one of them dies. At this moment GOG games don't fall into this critical category, as I still feel confident I can re-download them in case of such mishap.

The internet. I got one polish company having all games + extras backed up for me. And if they fail for some reason a multitude of users have the games backed up on their machines and available.

You beat me by mere seconds, sir.

And as some have suggested, at this point my main reason to back up GOG games is not for the case if and when GOG closes doors, but so that I have faster access to them in case or (re-)installation. With small files the download time does not matter, with bigger yes. I still have just a measly 10Mbit/s ADSL line at home. :(

With e.g. my DotEmu games, it is already a bit more about that "what if they close doors" case, even if they necessarily don't. I think I won't be even revisiting that site anymore much, so maybe at some point I will forget the whole site, and my account/password to it.

And in case the apocalypse really comes, I for one will not be sharing my GOG games to the lazy ones who didn't back them up, sorry. ;) Also, PBay is inaccessible from here (blocked by ISPs), so I can't really use it as my personal backup storage either, especially due to its trojans.

NAS with Raid 1 (mirrored) HD's.

At least 4 here and more there. ;)

Backup is easy: simply don't go through the motions of deleting the downloaded installation file(s). Set the gOg downloader to save them to some special place, and your work is done.

That said ; ) mine are backed up twice more: My Games under My Account, and a portable USB hard drive.

Home FTP server for easy access.

External USB HDD... and thinking of double backing now... hmmmm

I've thought about this before too and how hilarious it actually is.

I can just imagine me and my family somehow surviving an apocalypse of some sort, and me rounding up my wife and kids after a month of eating dead people to traverse the irradiated wasteland accross town to retreive my USD hard drive full of GOG games. LOL!