Chris Scholten created some cool and colourfull icons sets for Window Xp and Vista. The icon sets for Programs, File Association and Windows icons. The icons set are available for free downlaod. I had also write some other articles about icons are Cute Blogging Icon Set for Designers, 422 Mini Icons in PNG Format.
Visual Pharm is created some animal icons for your OS theme. The animals which they given are Butterfly, Dolphin, Elephant, Hippopotamus, Panda and Turtle. The icons are in png, ico and also available for Mac OS too. The size of the icons are 16 pixels, 32 pixels, 48 pixels and 128 x 128 pixels. The icon usage are free for personal or commercial use but they require linking to their web site.