Open Sans is a free, modern font that is perfect for any design project. It is a sans-serif typeface designed by Steve Matteson and commissioned by Google. Open Sans is optimized for print, web, and mobile interfaces,
From Wikipedia
Open Sans is an open source humanist sans-serif typeface that was designed by Steve Matteson under commission from Google. It was released in 2011 and is based on his earlier design called Droid Sans, which was specifically created for Android mobile devices but with slight modifications to its width.