Source Code Pro. Currently making the rounds on the Intarwebs:
Source Code Pro is a set of OpenType fonts that have been designed to work well in user interface (UI) environments. In addition to a functional OpenType font, this open source project provides all of the source files that were used to build this OpenType font by using the AFDKO makeotf tool.
Via Mikko Ohmataa, said post also having useful commentary and comments.