Open Sans

Open Sans is a sans-serif font. It goes well with Montserrat, Roboto, Brandon Grotesque, Lucida Grande, Bitter, Source Sans Pro, Publico, Helvetica Neue, Playfair Display and Georgia. If you're thinking about using Open Sans then try 28px for headers. Give 16px a shot for content.

People commonly tag it as design, tech, agency, app, portfolio, clean, photo, modern, simple and sans serif. You can get it from Font Squirrel, Google Web Fonts, MyFonts and Typekit.