Diablo 3's first expansion, Reaper of Souls. Invitations are being sent out now, so keep an eye on your Battle.net account to see if you got in.
This beta, which would normally only be available to friends and family of Blizzard, will be open to "valued media contacts" and 3,000 of Diablo 3's most active players. The beta requires no NDA signature, so participants are free to capture footage and stream as much as they want. We anticipate footage will be hitting YouTube as soon as tonight.
Blizzard has posted a lengthy FAQ on how to opt into the beta, report bugs and generally contribute toward creating the best expansion possible. There's also a specific process you'll need to follow in order to import your existing character, so be sure to read through the FAQ before diving in.