Buy this to get Retro FPS Prequel Demo (Ion Maiden)

Or this
and this - but it's probably obvious for everyone here on GOG.

Buy this to get The Raven: Legacy of a Master Thief Classic

Buy any of these to get Close Combat
Buy this to get Simon the Sorcerer Legacy Version
Buy this to get Simon the Sorcerer 2 Legacy Version

UPDATE
Maniac Mansion (Original & Enhanced)

Time-limit:
Pre-order BATTLETECH to get Shadowrun Returns

Time-limit:
Pre-order Pillars of Eternity II to get Wasteland 2

All owners of "Bombshell Digital Deluxe Edition" or "Bombshell Digital Deluxe Edition Upgrade" will find a bonus code for Ion Maiden's preview campaign in their account under More -> Serial Keys.

ADDED
Own GWENT and subscribe newsletter to get The Witcher: Enhanced Edition and GWENT Card Keg

That is true, I got the code for Ion Maiden preview campaign, when I got Bombshell Digital Deluxe Edition, as described on the Ion Maiden game page.

For those of you perhaps confused by the differing reports on the freebie Ion Maiden preview campaign that you can get if you own one of the Bombshell game versions, that is because GOG changed how and what was available.

When GOG first mentioned the bonus demo or whatever it really is or was, you need look no further than the bottom right of the Bombshell downloads page, where it was first mentioned as Retro FPS Prequel Demo (Ion Maiden), which was a 70 Mb zip file download containing both Windows and Linux versions. That was first mentioned here by kbnrylaec on February 28, 2018. That was what I saw and downloaded around then. That was it, and there was no new game listed in my GOG Library, and there was no Serial Keys entry.

Just recently, thanks to a friend making a query, I happened to revisit the Bombshell page in my Library, and noticed that a Serial Keys entry now existed. I wondered what that was, because I still had the Retro FPS Prequel Demo (Ion Maiden) link on the downloads page. On checking that link and the one available for the GOG Downloader, I discovered neither now work.

I decided to use that Redeem code now mentioned in Serial Keys, which by the way says nothing about Ion Maiden until you use it. Through that I not only got a new game page for Ion Maiden in my library, but also a 37 Mb exe file download. It appears to be a more recent update to the zip I originally got.

I suspect not many are aware of this change and update. See ATTACHMENTS below.

Time-limit:
Pre-order Warhammer 40,000: Gladius - Relics of War Deluxe Edition to get Warhammer 40,000: Rites of War

Time-limit:
Buy 7 Billion Humans to get Human Resource Machine

NOTE
This thread have moved to a new version. OP will not update any more.
Please follow the newer thread instead:
https://www.gog.com/forum/general/bonus_games_included_in_gog_games_v2

ADDED in newer thread:
Buy this to get Neverwinter Nights Diamond (You WILL NOT get another copy if you have owned the game.)