The only one I could find again is the System Shock 2 skin (below, in the middle). So much green Matrix text! So many video games! So many futuristic alien devices! So many anime babes! So many mock-Macs! So much rusty metal! I wish I could remember the names of skins I used, there was a beautiful teal one I'd love to see again. It's a simple website showing off a whopping 65,681 skins as you scroll down, down, ever down through ancient digital artefacts.
What a treat it is to scroll through the tens of thousands of skins on the Winamp Skin Museum and see the exciting, strange, colourful, and horny range of old Winamp skins.
In the late nineties and early noughties, no video game forum was complete without a 'post your desktop' thread, and no desktop screenshot was complete without a snazzy Winamp skin displaying your personality and illicit MP3 collection.