Posted May 19, 2017
GR00T: It's... actually just an Open Office Word file. I just add titles as I buy them, with a note where they were bought (GOG, Origin, Steam, Humble, etc). The DRM-free ones are all backed up on my backup drives.
Falkenherz: I do it the same way, though I use a spreadsheet file with the following columns: Game (here goes the title)
Source (GOG, Indiegamestand, Humble and so on, so I quickly know in which folder I put the installation files or from which account I can re-download them)
Genre (RPG, Puzzle, Platformer an so on; allows me to sort the entries to search for a specific game type if I am in the mood for a roguelike for example)
Played (to mark whether I played the game already; again allows for sorting so that I can better see my unplayed games)
Rating (my personal rating: ++ Favorite, + Good, o Ok, - Bad, -- Trash)
Might be some work at the beginning to record all the games you have, but once you are up to date, it takes only a minute to add the two or three new games you bought in a sale for example.