New answers tagged development
There are the GeoSilk icons that are used in GeoServer and I think UDig. They are released under a CC-by license.
These are the icons used in the latest QGIS (by default I think) and GRASS GIS: http://robert.szczepanek.pl/gis-icons-0.1/ They are licensed under the Creative Commons Attribution-Share Alike 3.0 Unported License which means you can use, alter and share them as long as you attribute the original creator and share your icons under the same or similar ...
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