Merriweather is a free, open-source serif typeface available through Google Fonts. It was designed to be highly-readable on screens. Unlike a lot of other serifs available on Google Fonts, Merriweather contains a full set of weights and styles — light, normal, bold and ultra-bold — all with matching italics.
If you are looking for an alternative to Merriweather, I recommend checking out FF Tisa and Chaparral. I think Open Sans would make a nice font combination paired with it.