It is nice to have great icons and I always try to download quite a few to freshen my desktop up whenever possible. There was a great resource for desktop icons on a website called pixelgirlpresents a long time ago however this no longer exists. After a quick google, a wide array of icons will appear to download and apply to folders on your computer. There is also great software available that allows you to create your own icons which you can then save out as icon sets for other users. The best software that I found was Icon Workshop (download here) which was very user friendly and gave fantastic results.