I'm currently wrapping up my hard difficulty/no alarms/no kills run through Deus Ex: Human Revolution. I actually bought this game for $10 two months before it was offered free to Plus members this month, but I certainly don't mind throwing a few coins at such an excellent game (I also bought all the DLC on sale and enjoyed The Missing Link quite a bit). By the way, it was announced yesterday that the Director's Cut version that was created for the Wii U is being ported to PC/360/PS3 sometime this fall, though I doubt I'll be double dipping.
Next, I'm having trouble deciding between Borderlands 2, which I bought three months ago, and XCOM, one of the new free games for Plus. I've never played a pure strategy game before, but I hear good things about XCOM and alien invasion stuff is right up my alley in terms of subject matter. I actually tried the demo last weekend, and it seems fairly straightforward, but I know it ramps up in difficulty and complexity quite a bit, even on easy and normal difficulty settings.
EDIT: but all of the above best laid plans could change tomorrow if tomorrow's Fry's ad offers a good deal on The Last of Us. Once in a great while they'll offer a new release game for $50 the week it comes out, but I'm not holding my breath--though regardless, I'll be driving by my Fry's Saturday morning for an appointment, so who knows? lol