The sequel to 2007′s Portal will find its way to the shelves and download slots of fans on the 18th of April (US). However Valve are trying to offer an incentive to pre-purchasing the game now by offering a 10% discount. If you buy the Portal 2 pack you will get 20% off.
Portal 2 will be offering players more of the same puzzle-solving that made the first game so addictive, but also throwing in a co-op mode where 2 people can complete missions together; these will undoubtedly be more difficult.
Amazon are also offering the game at the discounted price if you pre-order now.