The Legend of Zelda: Spirit Tracks (just couldn't get into it after a while)
Super Mario Bros. 3 (I know, I'm a monster)
Castlevania: Lords of Shadow (got bored)
Prince of Persia Trilogy (pushed through the first, played some of 2, still looking for motivation to finish)
Crysis (got bored)
Borderlands (practically done, but there's this one boss fight from the DLC that's just impossible in 2-player co-op)
That's pretty much it for now. I've been recently going back and finishing games I left for dead like Halo: Reach on Legendary difficulty on solo which I'm currently working on and I've also got back to Super Mario Bros. 3 but I feel like I'm ignoring it again, so I'll make sure to get back to that.