Basslet - the name sort of says it all - its a wearable sub unit that is wrist mounted and gives you the physical sensation of bass while wearing headphones.
Interesting idea, we've not tried it ourselves but the sales pitch on the Kickstarter campaign appears to to have wowed a lot of folks - with almost €600K raised, the unit is now available at a reduced price of £119 (from £179)
It gives what they are calling HD Vibration Technology which covers 10-250Hz range and comprised of a unit you plug between your device and your headphones, which transmits the signal to the wrist mounted driver for the tactile bass sensation.
We've seen wearable sub units before, but this has got to be one of the smallest. We assume that the brain translates the physical sensation of the bass on your wrist into an overall more visceral experience for the listener - the testimonials seem to suggest that it works anyhow.