It's not everyday I drop a post saying 'oh, look everyone, this random game is now available for you to purchase', but for Mark of the Ninja, I'll gladly make an exception. As someone who is currently about two thirds of the way through the game on Xbox LIVE Arcade, as someone who thinks this is a genuine game of the year contender, you have to play this game!
I don't think I've ever played a stealth game with so much polish. In the genre there's always a need to suspend disbelief, to sort of play with the game, to not exploit certain mechanics, to not simply kill everyone. Sometimes you have to make that active choice to get the best from stealth games. But with Mark of the Ninja, there's no need to condescend -- the design is so water tight, and the execution so perfect, that everything just works.
Mark of the Ninja comes highly recommended from me, you can check it out on Steam here .