There's a lot of typefaces created specifically for screen reading, but to effectively use them it's important to know beforehand whether the rendering engine of your client's device supports hinting, which is responsible for the optimized rendering of the letterforms in small font sizes.
The latest version of Liberation font family do have hinting instructions. Fedra is another great font that has versions tailored for print and screen.
http://www.typotheque.com/fonts/fedra_sans_screen