Windows: Rainmeter is hands-down the most popular tool for customising the look of your Windows desktop and it's hitting version 4.0 with some new skin creation tools.
The biggest new tool here is the shape meter. This allows you to create vector graphics, adding a whole new level of customisation for buttons and skins. From the looks of it, these can even be animated to suit a variety of needs. Skin creation is also simplified a bit, and a massive list of other smaller changes should help simplify other parts of the program. There isn't much for users here, but skin creators have some great new tools to play around with.