The rumormill spooled up on March 30, when Elon Musk tweeted a message promising a "major new product line" that wouldn't be a car, to be presented April 30, now just a week away. Speculation ranged from an electric motorcycle to a line of personal electronic devices, but those guesses now appear to be way off the mark. Guesses about a home battery system were closer to the mark.
In an email sent to investors and analysts, Tesla exec Jeffrey Evanson reportedly revealed that the company would be rolling out both a home battery and a large utility battery. The products look poised to leverage the battery technology Tesla has developed for its electric cars, but adapted towards stationary applications.
Full details won't be announced until the big event at the end of this month, but would appear to offer private and business users the advantage of Tesla's lithium-ion battery tech to store power generated by on-site wind turbines and solar panels and protect against power outages on a large scale. There is some good background on the battery system in this 2014 article from the San Jose Mercury News.