As unbelievable as it might seem, Adobe announced immediate availability of Source Sans Pro, their first open source font, under terms of Open Font License.

The new font family is a gothic sans, available in 6 weights, from ExtraLight to Black, and both regular and italics.

Note that Extra Light is absent here: Pango engine simply doesn't see it.

Source Sans Pro was designed by Paul D. Hunt with ATF's gothic fonts in mind and built with makeotf tool from Adobe Font Development Kit for OpenType:

...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. In thinking of typeface models that accomplish these tasks well, I was drawn to the forms of the American Type Founders’ gothics designed by Morris Fuller Benton. In particular, I have always been impressed by the forms of his News Gothic and Franklin Gothic, which have been staples for typographers since their introduction in the early twentieth century. While keeping these models in mind, I never sought to copy specific features from these types. Instead, I sought to achieve a similar visual simplicity by paring each glyph to it’s most essential form.

Just to illustrate the difference between News Gothic and Source Sans Pro:

The font family has an extensive support for Latin-based scripts, but sadly Cyrillic and Greek aren't available in this version. Adobe is planning to take care or that in the next releases, though. A monospace version is in plans (in fact, already being worked on, it appears).

You can download sources and OTF/TTF files here. Source Sans Pro is already used in few products such as Adobe's Webkit-based code editor called Brackets.