I've gathered a number websites which host free icons. They are all listed in this article.
Everyone is going google plus crazy, as they should it's a great app. Here are some icons to help you link to google plus.
FindIcons.com is by far my favorite site! Always find what I need and can easily see the license for commercial work. Great site!
26,000+ free icons, buddy icons, xp icons, vista icons, desktop icons, aim icons
Both premium and free icons at Icon shock, the above link is for the free.