I rarely customize my desktop with icons, but today I needed one for a special project. I went over to the Icon Factory, which has freeware for individual use as well as royalty-free images developers can license for use in professional applications.
Browsing the freeware collections, which date back to 1996, was a trip down memory lane. In addition to some tech nostalgia (remember eWorld?) I found icons for Larry Niven's Known Space books, and icons for the cult SF TV show Lexx. Plus lots of Star Trek icons. My favorites for visual impact are the eye-catching Smoothicons done by the Icon Factory team. And Kate England's Private Eye icon sets (there are three of them) turned out to be irresistable for a mystery writer.
Icon Factory recommends getting Pixadex, which organizes your icons the way iPhoto organizes photographs. The program is $18.95, but there's a free demo version.