I keep a majority of the games I buy just for this reason.
Though the ones I've found myself killing time with lately are
Dark Souls I bought it a looooong time ago, beat it once obtained all the items, experienced all the content and was done with it. But after reading so many posts about how great it is blah blah blah, I figured I'd give it another go and mop of the rest of the achievements I missed. and wo and behold I played NG+,NG++,NG+++, caved and bought the expansion and you guessed it lol NG++++ (fighting Ol' Al, Kala and sanctuary Guardian at this point was just BS :B) and then a NG+++++ just for fucks given and another stab at the DLC. Seems I was claimed by the abyss for a moment there lo.
Bioshock 1-2. They're memorable enough to make you nostalgic and the right length to merit a replay and not become a chore.
Dragons Dogma Dark Arisen There's so much endgame content and a friendly NG+ that there's little excuse not to boot it up and try to farm those damn Cursed weapon/armor/item level 3s! I want my ninja armor damn it!
League of Legends an unhealthy relationship this one.
Civilization:Revolution compared to the PC members of the family playing this may be blasphemy but I don't care. I have fun with it and the scenarios add a refreshing spin to it.
Whenever I dust off the PS2 the games I immediately harken to are
Rogue Galaxy Incredible game, great end game content and a sneaky NG+ also (Always forget to press the buttons to trigger it lol, starting from scratch each damn time -.-)
DQVIII I just harped about this one on another thread, but yeah. Can not go wrong
GodHand This! This is my go-to when I turn on the PS2. It's a blast. Wished more people played it or knew about it.
Shadow Of the Collosus Fan Favorite. And very accessible If you know what you're doing, can finish a NG+ in one sitting. Makes for easily justified replays.
Okami The only Zelda-like I bother playing, I know there are die hard Okami fans on the Escapist, so I can spare myself the explanation why I would replay this.
Anything by Level-5 and formerly known as Clover I seem to have a pattern going on.
There's a shit ton more, but no one wants that.