We currently use Arial in regular and bold weights on the site. Sometimes we use Open Sans on buttons and titles, and we also use other fonts to create interest or to complement specific brands on site.
We are moving towards using Roboto and Noto because it serves user needs very well around the world. We've introduced it in the new Login/Register area. Currently, the scope is body font to start with, but over time it will be used across the site.
Downloads and more information on Roboto and Noto are available on material.io.