Well, you guessed Thief, but let me confirm it; it does have a lot of non-lethal options. In fact, on the harder difficulty levels you are not even allowed to kill people. In fact it does stealth quite a bit better than Deus Ex IMHO, and in at least one level you are not even allowed to let people see you.. One of my favorite series of games of all time and I can't recommend it more highly.