ActiveSkin is a UI development environment lets you create complex interface 'skins' with non-rectangular windows, shadows, morphing, animations, and semi-transparency. Skins can be saved in the application or over the Internet. With ActiveSkin you can make your program look exactly like it was from another OS, or create your own WinAmp-like 'skinned' interfaces, or just give it a different, distinctive look.
More info:
hxxp://www.softshape.com/activeskin/