Fira Sans looks nice. It also comes in monospace, and even has 16 different weights.
Read about it at
http://www.donotlick.com/2014/05/23/try-out-fira-sans-a-free-open-source-typeface-commissioned-by-mozilla/
Preview and download 3.1 at http://dev.carrois.com/en/fira-3-1/#download
Find older versions at https://github.com/mozilla/Fira and https://github.com/mozilla-b2g/moztt (which also has other free fonts)
