It would be nice to have U+202F NARROW NO-BREAK SPACE in Source Sans Pro. This is a blank glyph, usually a bit narrower than a normal space.

U+202F is used in French typography (before some punctuation marks), although many people substitute either a normal space or no space. It is also used in Mongolian.

See http://en.wikipedia.org/wiki/Space_(punctuation)#Spaces_in_Unicode for more details on this character.