If you've somehow never tried Valve's stupendous Portal 2, which was released a little over a year ago, you have no excuse anymore: the game is on sale for only $6.79, down from its usual $19.99 price point. It's available on PC and Mac through Valve's Steam client, and the timing couldn't be better — Portal 2's level creation tools, known as the "Perpetual Testing Initiative," are available today as well. While some have dinged Portal 2's replayability, the first time through is about as much fun as you can have with a video game, and the new DLC means there will be new chambers to test for years to come. It's hard to go wrong at this price, especially with the flood of community-generated content that's sure to be coming. The deal's only good until May 10th, so don't delay — there's Science to be done.