Free keys - every last letter or number is replaced with X with the letter it should be for redemption next to it (no idea if there's sweepers on these boards).
Mafia 3 (GOG)
KY7TB6D679DF784D4X - Last 'X' is 0 (zero).
Nomad Survival (Steam)
ACCNG-RM86A-KR2IX - Last 'X' is D.
Distant Worlds 2...