On the Raspberry Pi there is a very nice feature that allows to create very cool shaped windows: the control of the opacity of the 3D window.
How to Control the Window Opacity on the Raspberry Pi

3D Programming, Prototyping, Pixel Hacking and Gamedev with GeeXLab Proto-Engine
On the Raspberry Pi there is a very nice feature that allows to create very cool shaped windows: the control of the opacity of the 3D window.