Source Sans Pro: Adobe's first open source type family

Source Sans Pro was designed by Paul D. Hunt as Adobe's first open source 
typeface family, conceived primarily as a typeface for user interfaces. 
Source Sans Pro draws inspiration from the clarity and legibility of 
twentieth-century American gothic typeface designs.

Source Sans Pro currently supports a wide range of languages using Latin 
script, and includes all the characters in the Adobe Latin 4 glyph set. As 
an open source project, it is expected that incremental updates will be 
made over time to extend glyph set coverage and functionality.

WWW: http://sourceforge.net/projects/sourcesans.adobe/