The 1.6.x branch of PrestaShop is the next version of PrestaShop.
It is currently under development. You can see it get better (and participate!) here: https://github.com/PrestaShop/PrestaShop/tree/1.6
Here are the 1.6.x branch release dates:
PrestaShop 1.6 alpha 1 (v184.108.40.206)
November 20th, 2013
|PrestaShop 1.6 alpha 2 (v220.127.116.11)||December 19th, 2013||n/a (changelog)|