Tuesday (6/30) at about 11 AM PT the Lindens announced the partial release of Experience Tools. I write ‘partial’ because they are only fully released to Premium Members. This afternoon the 3.8.0 (302622) Jun 12 viewer automatically downloaded and I installed it.
This is also the main viewer now. Look at the download page.
That is a Jul 2, 2014 video.
The announcement is here: Experience Tools Available Now For Premium Members!
All users can get the viewer and use it. My understanding is only Premium Members can build experiences with this viewer, but all users can enjoy the experiences.
There is a knowledge Base article that helps you get started creating experiences. See: Experiences in Second Life. This is for both those wanting to participate in an experience and those wanting to create an experience.
There is a forum section the beta testers and early adopters used. It is now open to all. See: Experience Tools Forum.
You can find experiences to play in Second Life™ here; Resident-created experiences in our Destination Guide and the Linden Portal Parks.