http://www.kde-look.org/index.php?xcontentmode=22x27 Icons at kde-look
http://www.gnome-look.org/index.php?xcontentmode=120 Icons at gnome-look
http://www.everaldo.com/crystal.html Crystal icons — an icon set often used in KDE desktop environment
http://tango-project.org/Tango_Desktop_Project Tango Desktop Project — an icon library for KDE and Gnome, licenced under Creative Commons Attribution Sharealike
http://www.famfamfam.com/lab/icons/silk/ Silk icons — 1000 icons licenced under Creative Commons attribution license
http://www.openclipart.org/ Open Clip Art Library — a library used for some Mediawiki icons, mostly not suitable as a source of icons but rather as of clipart
http://commons.wikimedia.org/wiki/Category:PD_OpenClipart Open Clipart at Wikimedia commons