Source Sans Pro is an open-source grotesque typeface designed by Paul D. Hunt for Adobe Systems. The design was inspired by American gothics, however, some traits such as the tail on the lowercase “l” are more reminiscent of humanist typefaces. The slightly condensed letterforms of Source Sans Pro make it ideal for user interfaces where space is a concern. It’s available in a full set of six weights with corresponding italics.
If you are looking for an alternative to Source Sans Pro, I recommend checking out Open Sans and Alright Sans. I think Merriweather would make a nice font combination paired with it.