All my games I haven't played were either free gifts from Steam/Friends or from humble bundle when I've wanted one game and the $1 has been worth that one game.
This or I have them on Origin / other platforms that I've played.
Some have been part of bundles through steam aswell. Some are test branches (Dota test)
Some are games I completed before Steam even existed (Half Life and all variants of) and some other games I managed to add via CD Key so I could get rid of the physical copy.
I even have duplicates for some reason (San Andreas)
Some of them I dont even know how they are on my games list.
Some of them were actually free for limited amounts of time.