WinCustomize.com
Possibly the largest site dedicated purely to windows UI customization, WinCustomize is home to over 66,000 user-submitted uploads, covering nearly 50 applications and UI elements. The site also boasts a large and helpful community, providing assistance to fellow users via forums and IRC. This is the essential site to have in your bookmarks list if you're interested in skinning.
Additionally, the site has launched a resource and help wiki at http://wiki.wincustomize.com It is currently the most comprehensive skinning wiki in the community.
Customize.org
One of the oldest customizations sites on the Internet, Customize.org caters to more than just Windows customization, though that represents the bulk of their content. Recently relaunched with a Web 2.0 style look, Customize.org is a growing site with a very active community.
WinMatrix.com
While WinMatrix itself does not host any customization files (icons, wallpapers skins etc.) they have one of the most robust, active and friendly forum communities out there. If you're looking for feedback on your latest creation, or general technical assistance, WinMatrix is a fantastic resource.
DeviantART
DeviantART is one of the biggest digital art sites on the Internet. The site has everything from photography, to oil paintings, to sculptures and digital art. What many don't know is that the site started as a Windows UI customization gallery. They still have a very active set of galleries dedicated to icons, skins and wallpapers. While there's a lot to sift through, you're sure to find something you like there.
Skinning.net
A smaller site, more focused on community than content, Skinning.net is a great starting point to ease yourself into the world of Windows Skinning. The community is friendly and extremely helpful, and a good place to showcase your early work. They'll welcome you and give you good advice on improving your work, or where to get the best content for your PC.
Skinartistry
Another community focused skinning site. Skinartistry is one of the best sites for tutorials and help on how to skin your PC. From user-focused tutorials, to how-tos on designing specific elements of a desktop user interface, SA is a must-visit spot for anyone looking for help on how to get into skinning.
Bonus: Community Suggestions
Based on feedback from the skinning community, I've thrown in this entry for "Community Suggestions" These are all smaller skinning sites but worth checking out. WinModify specifically is about ready to undergo a relaunch with a new design, so keep your eye on them!
1. Aqua-soft.org (very cool community going here)
2. JoeJoe.org
3. SkinBase.org
4. WinModify.net
5. Adni18.com
6. LotsofSkins.com
7. Crystalxp.net
8. AeroXP.org
9. CustomXP.net
10. GetSkinned.org
11. GUIStyles.com
12. Interfacelift.com
13. OSX-e.com
14. oxyx.oxygen-inc.com
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