Open Sans is an open-source, humanist sans-serif designed by Steve Matteson and released in 2011. The five weights with matching italics make Open Sans extremely versatile and useful for a wide range of applications. It is a ubiquitous font on the web, used by everyone from Google to WordPress. I’ve even heard it referred to as the “flat design” font. I like to think of Open Sans as the new Arial.
If you are looking for an alternative to Open Sans, I personally recommend checking out Source Sans Pro and Alright Sans. I think Merriweather would make a nice font combination with Open Sans.