Digital distributors tend to chuck around Metro 2033 deals like a game of discount hot potato, which has temporarily come to rest over at GamersGate. This apocalyptic FPS frustrated many gamers with its gruelling difficulty and awkward stealth, but delighted many others with its consistent vision of a subterranean society. Dave Brown voted it his Game Of The Year back in 2010, so it's well worth checking out if you enjoy a challenge. Note that you'll get a Steam code (and you'll already own it if you bought the Humble THQ bundle).
Thanks to bandwidthbob @ HUKD!