If, like many people nowadays, you're a social networking addict then Free 3D Social Icons will appeal to you. It's a collection of icons celebrating sites such as Flickr, Twitter and Facebook.
Free 3D Social Icons are available in several different formats and sizes, from 16x16 to 512x512 pixels. To change an icon, CMD-click to reveal the 'Information' dialog and then copy it to the clipboard. Then do the same with the icon you want to replace it with and paste it in. Alternatively, you can use an icon manager, such as CandyBar.
If you just can't get enough social networking in your life, then these icons provide a way of bringing them even closer to your Mac