Our second (!) Deal of the Day is Portal 2.
What better way to mix up our Deal of the Day with one of the best games ever? Valve are selling Portal 2 at a heavily reduced price, down from it's usual $19.99 price down to just $6.79. Remember that the new free DLC also came out today, Perpetual Testing Initiative. A two-for-one day if you ask me!
Portal 2 is available for both the PC, and Mac, and can be downloaded directly through Steam, or here.