Blizzard

Pre-ordering Legacy of the Void will net you early access to the Whispers of Oblivion prologue missions.

In standard or digital deluxe edition.

The prologue is split in three parts, serving as a good entry point for the series

Each faction gets powerful new units that can make the difference in battle.

Blizzard intends to slowly bring more people in to the beta, but you don't need a powerful PC, or even older versions of Starcraft 2, to get in.