Many Open Source projects provide their icons for use in other projects. In addition, the licenses of many icon sets allow for remixing and extension. This is hugely beneficial to end-users, as it allows designers to distribute add-on packs without having to worry about complex licensing terms.
Tango Icon Library by the Tango Desktop Project
License: "The icons in this repository are herefore released into the Public Domain."
The Tango icon set, designed as a proof of concept for the Tango style guidelines, contains a basic set of icons for common usages in the desktop environment.
Nuvola Icons by David Vignoni
The Nuvola icon set by David Vignoni was originally intended for desktop environments like KDE and GNOME. The final version contains almost 600 icons.
Silk Icons by Mark James
This free icon set by Mark James contains 1,000 16×16 pixel icons in PNG format. It includes emoticons, document, and application icons.
Silk Companion 1 Icons by Damien Guard
This icon set by Damien Guard extends the Silk Icons set with 460 additional icons.
Silk Companion 2 Icons by Chris Cagle
License: Creative Commons Attribution 3.0 b
Chris Cagle created an additional 64 icons to further extend the functionality of Silk Icons.
Sweetie BasePack Icons by Joseph North
The Sweetie BasePack has about 100 small icons in PNG format; PSD are included in the download file.
Sweetie WebCommunication Icons by Joseph North
The Sweetie WebCommunication has 37 icons related to communication. Feed, email, and speech bubble icons are included.
Mini Icons by Mark James
License: "Available for free use for any purpose."
144 icons in GIF format. This is a pretty basic set of file, folder, and doctype icons.