Source Sans 3
Source Sans Pro, Adobe's first open source typeface family, was designed by Paul D. Hunt. It is a sans serif typeface intended to work well in user interfaces.
Merriweather
Merriweather was designed to be a text face that is pleasant to read on screens. It features a very large x height, slightly condensed letterforms, a mild diagonal stress, sturdy serifs and open forms.
Charis SIL
Charis SIL is a Unicode-based font family that supports the wide range of languages that use the Latin and Cyrillic scripts. It is specially designed to make long texts pleasant and easy to read, even in less than ideal reproduction and display environments.
Andika
Andika is a sans-serif font family designed and optimized especially for literacy use. It supports almost the complete range of Unicode characters for these scripts, including a comprehensive range of diacritics and a large set of symbols useful for linguistics and literacy work.
Atkinson Hyperlegible
Atkinson Hyperlegible, named after the founder of the Braille Institute, has been developed specifically to increase legibility for readers with low vision, and to improve comprehension.