Lato is an open-source, humanist sans-serif typeface designed by Polish designer Łukasz Dziedzic in 2010. Lato was originally designed for a corporate client, but after the client went in a different stylistic direction, Dziedzic decided to release the font to the open-source community. Lato is available in five weights — thin, light, normal, bold and ultra-bold — each with matching italics.
If you are looking for an alternative to Lato, I recommend checking out Open Sans and Alright Sans. I think Plantin would make a nice font combination paired with it.