I used to use a rather basic, but effective system of spreadsheets which included last played version number, date, and a general rating. I also played around with writing short summaries once since I almost always forget about stories and characters due to dev times. However, I quickly gave up on that due to laziness.
Tho, now my process is MUCH easier. I see game, I download game. I extract game into game folder. I play for like 10 minuets, decide to try something else. Later, I forget I have game and redowload it, get too lazy and leave it in downloads folder. I open the game, realize I played it before and x out and do something else.
Eventually, download folder is too full, so I delete everything. Then one day I realize I have 20-30 games in the game folder that I don't remember doing, so I just delete them all assuming they're all out of date.
Then the cycle repeats itself. Tho, sometimes I do break the cycle by accidentally playing a game completely, or replaying a game I haven't played in a long time that I know I used to like.