A short list of body fonts that render consistently well in smaller sizes:

  Open Sans
  Source Sans Pro
  Droid Sans
  Lato
  Pontano Sans
  Proxima Nova
  Museo Sans
  PT Sans
  Segoe UI
  Tondo 
  Istok Web
  Interval Sans (Light)

http://www.google.com/webfonts/specimen/Source+Sans+Pro

http://www.google.com/fonts/specimen/Droid+Sans

http://www.google.com/fonts/specimen/Lato

http://www.google.com/webfonts/specimen/Pontano+Sans

https://typekit.com/fonts/proxima-nova

https://typekit.com/fonts/museo-sans

http://www.google.com/fonts/specimen/PT+Sans

https://www.microsoft.com/typography/fonts/family.aspx?FID=3...

http://www.webink.com/face/1133

http://www.google.com/fonts/specimen/Istok+Web

http://www.fontspring.com/fonts/mostardesign/interval