Designed by Paul D. Hunt, Source Sans Pro was created for use in user interfaces as well as text typography for screen and print, and was inspired in part by the gothic (read: sans serif) typefaces of Morris Fuller Benton, who designed News Gothic and Franklin Gothic. Other design features, like the differentiation between the number '1', the lowercase 'l', and the capital 'I' (something which has always bugged me with sans serifs), were suggested by user feedback.
|Differences between commonly confused characters: 1, I, and l|
Source Sans Pro includes six weights in upright and italic styles and is available in OTF and TTF at the Source Sans Pro download page on SourceForge.
Seems like there's no better way to learn about sans serifs than to mess around with one. Although my first thought was, "Maybe someone will design a companion serif for it?" Open source, of course.