About half the games on my shelf. Every time I try to get back to something, I run out of time before something new that I've been waiting on comes out.
I just finally had the free time to get back to playing Bionic Commando and Borderlands, again; just finished a Good runthrough of inFamous 1/2, and an Evil run on the first. Hell, it took me a year to even crack the cellophane on God of War III (for which I bought the Collector's Edition, Day One).
It's not so much that I have a hard time getting back into a game I keep promising myself I'll go back to, it's that I simply don't have the time. I piddle around in games and take my sweet time with them. Between those two factors and my work schedule, it makes for little headway on the time I do spend gaming, so I constantly rack up more titles that get put on back-burner. I can even think of about a dozen off the top of my head that I actually have yet to pick up, but want.