Source Sans Pro

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

The primary need for type in Adobe’s open source applications has thus far been for usage within user interfaces. A second environment of perennial interest to Adobe is the realm of text typography. Thus the immediate constraints on the design were to create a set of fonts that would be both legible in short UI labels, as well as being comfortable to read in longer passages of text on screen and in print.

You can download it here.