DigitalGarage -- Carbon fibre and chrome...
KDE2 -- More KDE icons
WindowMaker -- Icons by the Windowmaker project
KDE1 -- List of icon sets from the KDE site
KDE3 -- Independant KDE icons
DeviantArt -- DeviantArt's galleries of icons
While many of these icon sets come from the various linux projects, icons are standard things which should work equally well in any application. See the Graphics programs section for software if you need to convert any icons, and the Compression programs section for reading any of the compressed files