This is the sheer power of Steam sales -- I am considering buying a game I already own on another platform. A game I have played and finished. A game I know in my heart of hearts I will never even download, let alone play. This is outside the realms of common sense, or any sense for that matter.
The game is Batman: Arkham City. $7.49 is a pretty ridiculous price considering I just paid $12 for a salad and some fruit juice.
Civilization V is also current 75% off $17.49, and Dishonored is 50% off at $39.99.
If I could make one more recommendation, however, it would probably be Mark of the Ninja -- a genuine game of the year contender for me. It's currently 50% off at $7.49.