The Ys games all have awesome soundtracks. While I wouldn't call all the songs in them ambient or chill-out there are also quite a few songs that definitely fit the description. Like The Guidance of a White Tower or Water Prison (starts at 23:57) from Ys - Origin. Click on more in the description on YouTube to get links to jump to the beginning of all songs. Genesis Beyond the Beginning (6:25 or 1:28:36 for the alternative version) is also super awesome but perhaps not ambient enough for your taste.

The same can be said about the Turrican II soundtrack. Lots of great songs, some definitely great for ambience, some probably too fast. The very definition of ambient music of probably The Great Bath (22:07) from that soundtrack. Freedom (50:53) is also very ambient.

The above link to the Turrican II soundtrack is to the original Amiga version of the music. There was also a great rearrangement of the music released on CD which you can listen to here.

Machinarium, Botanicula, and anything else from Amanita Design might be what you're looking for.

Ah thanks for this!
I actually have Botanicula, haven't played it in a very long time.
So Machinarium should have also a relaxing vibe? hmm... might get that during summer sale, cool :)

Oh, and then there's Sunless Sea. I'm not entirely sure the music's relaxing. Disturbingly soothing?

The original Age of Wonders has some incredible music : https://www.youtube.com/watch?v=9o-7rPnZhXo&list=RD9o-7rPnZhXo#t=13

Here OP

Also here

Here aswell

Thank me later.

When you are not in combat the Skyrim soundtrack has some very smooth and ethereal tracks done by an orchestra. Install some mods to make the Auroraborealis look even more stunning and you might have a winner. You could even install a no combat mod or something to that effect and just roam the plains listening to the orchestra.

Wohoo! :)
Limbo... don't know about the game but that is pure ambient ;)

Obrigado!

flOw and Ico are console-only I think?

by the way... the third track reminds me of Boards of Canada, which is not a bad thing! ;)

Limbo is all about that ambient. And I like it very, very much.

Yeah, unfortunately Ico and this specifiic version of Flow (there was a browser version first) are on the Sony's hands.

Boards is good indeed.

ole hyvä (did I got it right?)

;)

Hehe... yes... You got it! :)

I'm getting Limbo then - probably from Steam summer sale when it starts :)
Ah, Sony's grip - not easy to unravel :P

Try Space Pirates and Zombies and I second FenixXP's suggestion of Endless Space and Endless Legend. You might also look into Deadlight.

Yes, Machinarium has a bit of a different vibe, but still a relaxing one.

I think some paradox games have that sort music like EU, CK, Stellaris,...

Cool. Thanks!
I have to think if I can find anything to add here it's been such a thrill :)

Stellaris... is that the new space game?

Yes, but it's not on gog yet.