I just have two folders. One for games that I haven't completed and other one for completed ones (or which are going to be updated). For games that I didn't care about much I delete everything and make a notepad with the game name to make sure I don't download the same game again.
Just make a database. Game title, maker, engine, a few notes about what you liked in it... Oh, include links, so you can check back on it later if it's still indevelopment.
I remember the good ones. Sometimes I end up re-downloading something, start it up, play for a bit, and then remember that I don't like it. I usually remember it again after that.