How-To:

• Paste keys in comments using the <spoiler> tag for easier browsing.
• Registering a game you already own invalidates the key.

When buying games, please prefer DRM-free platforms/portals:

• gog.com
• itch.io
• groupees.com
• Fireflower Games
• humblebundle.com/store
• Show Me The Games (collects dev direct distribution links)

Note that, sadly, less and less products on Humble and Groupees come DRM-free.

Being graced with a broken installation of the Steam client most of the year made me value the alternatives even more. At some point with the new update it started working again … time to finish all those hidden object games.

Also of interest:

• Steam Gifts community key exchange
• SE Steam group