The newest feature on GOG.com lets you import some Steam games to your GOG account for free.
If you've ever wished that you had bought a game on GOG instead of Steam, there's a chance you might be able to have both. CD Projekt is rolling out a new feature called GOG Connect that will let you link your Steam account with your GOG account. Once you're linked, you can import some games from your Steam account over to GOG.
Obviously, the list of games that this will work for is limited. There are currently 22 games available for import, and the GOG Connect page notes that "The eligible games are limited-time offers made possible by participating developers and publishers, so stay tuned for more games to come." The 22 games listed on the service right now all expire in 5 days, but once you import the game it will remain in your GOG.com account even after the import window has closed. Another caveat is that you can only connect one Steam account to your GOG account, and that connection is permanent.
There are some notable titles on the list of games you can import, like The Witness, FTL: Faster Than Light, Galactic Civilizations 3, and The Witcher: Enhanced Edition. You can see the full list of available games on the GOG Connect page.
You can get more information about the service by reading the GOG Connect FAQ.