Hi Metin, if you set the construction plane to the view direction then the rotate command would rotate in screen space. There's a script here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2599.8

or this variant:
http://moi3d.com/forum/index.php?webtag=MOI&msg=10112.9


> It would be very convenient and powerful if this would be possible, preferably
> with the addition of a 'trackball' rotation mode, rotating the object across multiple
> axes at once by moving the pointer around.

This style usually isn't done in a CAD program because it is imprecise to just map some arbitrary amount of pixel distance to an angle.

- Michael