Cue is a public domain gestural icon system which focuses on legibility and symbolic representation.

The Noun Project collects, organizes and adds to the highly recognizable symbols that form the world's visual language, so we may share them in a fun and meaningful way.

… an easy to implement framework of Google+ styled buttons, icon buttons & dropdown menu buttons

[dih-fawlt] [ahy-kon] is a repository of elegant, visually unified, minimal icons with crystal clear clarity, based on black color. It Can be used for UI design on web, portable devices (iPhone, iPad, Android compatible devices, other smartphones etc) , desktop applications, and generally any kind of electronic or mechanic machinery interface.

is a set of 434 (and growing) web and application icons inspired by advertising clip art of the 1920′s and 1930′s. The set is available in vector format and also includes a CSS/PNG sprite version. → GPL Licensed Version (download) : http://iconify.it/?file_id=9