Beta of Souls
Along with a small Friends and Family Beta, Blizzard have decided to invite 3,000 of the most active Diablo III players to check out Reaper of Souls. This Friends and Family Beta is happening now, and a closed beta should be happening by the end of the year.
The beta will let you play all of the expansion right up to the end, but you won't get to fight the final boss. If you want to be a part of the closed beta, you'll have to opt in from your Battle.net account.