Extension written by Donal K. Fellows which allows arbitrarily shaped windows on X11 and Windows to be created. MacOS and MacOS X are not yet supported.
http://www.cs.man.ac.uk/~fellowsd/tcl/#scripts/shape
Something like this needs to be combined with a 'drag and drop' extension to allow proper dragging of arbitrarily shaped objects from, e.g. a canvas or a text widget (where you might want to drag a non-rectangular block of text).
Does anyone have plans to really add proper 'drag and drop' support to Tk? Any TIPs in the pipeline?