The next (and hopefully final!) beta release for Drupal 8 will be beta 16! (Read more about beta releases.) The beta is scheduled for Thursday, October 1, 2015. To ensure a reliable release window for the beta, there will be a Drupal 8 commit freeze from 00:00 to 23:30 UTC on Oct. 1 (later on today).
Beta 16 will include a couple of important changes, including the removal of the ! placeholder from t(), and the moving of vendor code from /core/vendor into /vendor.