The Dark Mod is a fan-made spiritual successor to that style of gameplay, running on id Tech 4 (the engine of Doom 3) and carrying on its overall style very well - the engine lends itself to moody lighting and surprisingly sophisticated enemy AI, and while the physics are slightly clumsy, they capture the feel of classic Thief almost perfectly. Things are changed up a bit, though; the lockpicking is more of a timing-based minigame and there is no running storyline (unsurprising, as most of the levels are standalone fan-made missions). Still, fans of Thief's gameplay should have a lot to like here; there are dozens of fan-made missions available, and all of it is, of course, completely free.