Deadbreed. Noticed it in my inventory the other day and had no idea what it was or how it got there. It's a free to play game, but I never installed it so I don't know why it's attached to my account...
Was about to buy Outlast, then I saw I've owned it since it was released. For some reason I also own A Whispered World original and Special editions. No idea why. Just goes to show you how many games we don't play.
For me it was Tomb Raider. I wanted to buy it on this Eidos special that is going at the moment, and it said I own it already. I went to check my Library, because I was sure it was a mistake.