Roboto is an open-source, grotesque sans-serif designed by Christian Robertson in 2011 and released through Google. Despite being accused of being a “frankenfont” mashup of DIN and Helvetica, I’m personally quite fond of Roboto. I think it renders crisply on-screen and is very legible, even at smaller sizes. Roboto is the default font used in Google Maps and Google+.
If you are looking for an alternative to Roboto, I recommend checking out Benton Sans and DIN. I think Roboto Slab would make a nice font combination with Roboto.