While StarCraft 2: Legacy of the Void won't be out until this Winter, you can play a prologue of the expansion starting today.
Blizzard has announced that pre-purchase of Legacy of the Void is now available at $39.99. Those who order through Battle.net will immediately be able to play three new missions titled "Whispers of Oblivion" that tie the events of Heart of the Swarm and Legacy of the Void together. In addition, pre-orders at select retailers will allow players to participate in the Legacy of the Void closed beta where units and buildings are still being tested for balance.
StarCraft 2: Legacy of the Void is now available for pre-purchase on PC and Mac. Blizzard is offering an interesting incentive to players who throw down money for the standalone expansion pack: a bonus set of missions you can play right now.