It's legit free forever, it went free on Steam a few months ago or so.

I was worried about the guys that developed Sang-Froid, but people seem to say that it's just a show of good will for gamers, while they develop the sequel.

Gotta respect devs like these.

GOG key in Humble Bundle:
85% off The Witcher: Enhanced Edition Director's Cut
85% off The Witcher 2: Assassins Of Kings Enhanced Edition
75% off The Witcher Adventure Game
50% off The Witcher 3: Wild Hunt - Game of the Year Edition
50% off The Witcher 3: Wild Hunt - Expansion Pass
50% off The Witcher 3: Wild Hunt - Hearts of Stone
50% off The Witcher 3: Wild Hunt - Blood and Wine
40% off The Witcher 3: Wild Hunt

GOG itself:

Civ games on sale by themselves? Wonder what's up with that?

Sang-Froid being free is no longer unannounced. :)

I appreciate your dedication in posting these, but I'd kind of like to keep this thread solely dedicated to (unannounced) time-limited discounts on GOG itself, rather than GOG codes that one can buy at a discount elsewhere, or games available for free permanently. : )

I updated the OP to clarify what the qualifications are for discounts to be posted here. : )
(Please note that the discounts mentioned after the new edit [of the OP] are from the original version of the post, and are irrelevant except for "historical" purposes.)

There's still no announcement for the Civ sale. Does anyone have a hint how long this sale will last?

I see.

GOG announced the permanent free too late.
When @erbello reported it, we do not know it will be permanently free. :-P

That's cool -- I don't really mind that kind of thing, or the other "I posted it here, then it got announced an hour later" posts, because we don't know if it WILL be announced, and therefore don't know how long the discount might be good for, so better to let people know so they can take advantage while the discount lasts. I just wanted to make it clear because a couple other people have posted off-site promos and such in here before, and I didn't want it to become a trend. :)

Seems with today's Cossacks 3 DLC release there are a few unannounced discounts:
Cossacks 3 - 40% off (record low)
Cossacks 3: Digital Deluxe - 40% off (record low)
Cossacks 3: Guardians of the Highlands [DLC] - 20% off (first discount)

NOW they announced the Civ sale o_O

Announced.