ICONlook offers a pretty handy interface for searching and downloading icon files, whether for replacing out-of-place icons on your system or adding some graphical polish to a site. The engine's reach is somewhat limited at this point, but it helpfully provides a link to the license type for each result, when known, and seems to lean toward free and Creative Commons sources to begin with. Until Google adds icon files to their filetype:x capabilities, ICONlook is a good bet for designing your desktop.