Biofeedback has been around since the 70s, and is used in therapy for relaxation, even though its efficacy isn't exactly proven. myBrain Technology's Melomind piggybacks on this idea, with an EEG headset that connects to a smartphone. The user is meant to wear the headset, put in a pair of earphones connected to her phone, close her eyes and relax.
The stress measure is indirect, to avoid feedback loops (it's possible to be stressed yourself out by worrying if you're relaxing an optimal amount). The EEG measure hasn't been compared to the standard gear used in the lab, says Yohan Attal, one of the company's co-founders. Those studies are in the works, he says.
The headset is in preorder and costs $299. In the absence of evidence, I can't recommend splashing out for it — meditation is free, after all, and pretty relaxing.