I'd like to share with you very sweet and exciting example of UI features and UI usability for any 3D-capable app.
It is MandelBulb (a 3D version of a Mandelbrot set).
It is written on top of OpenTK. It took just 3 seconds to download pre-compiled binary and to run it with mono in my Ubuntu Linux.
Particularly I am very excited with its "3D Zoom Box".
PS Source code is also available.