Mark of the Ninja

Oh my gosh, this game is awesome. It's easily the best stealth game I've played in a decade. Even with a linear level progression, each level is so well crafted that there are multiple different ways to play it, and they're all fun and valid options.

And the game rewards your creativity. Each event rewards points, so sneaking by an enemy, terrorizing them, watching them stroll past a hidden body, all earn points. So playing the Path of Silence and completely bypassing every enemy you come across is just as valid, and just as rewarding, as playing the Path of Nightmares and turning each room into a complete killing zone of freaked out guards shooting each other.

It's availible on Steam and the XBox Live Arcade, and if you're at all a fan of stealth games, you really need to give it a try.