Hint:
Use the <spoiler>
tag for easier browsing.Steam keys usually don't wear off, but registering a game you already own invalidates the key.
DRM-free platforms/portals:
gog.com (Ack!)itch.iogroupees.comFireflower Gameshumblebundle.com/store (or not)Show Me The Games (collects dev direct distribution links)
… there are several new platforms now but those weren'T DRM-free, right?
Also of interest:
Steam Gifts community key exchangeSE Steam group
More info and Steam alternatives in the extended.