[kovidgoyal]

Here you go:

#!/usr/bin/env python
# vim:fileencoding=UTF-8:ts=4:sw=4:sta:et:sts=4:ai

__license__   = 'GPL v3'
__copyright__ = '2010, Kovid Goyal <kovid@kovidgoyal.net>'
__docformat__ = 'restructuredtext en'


from calibre.customize import InterfaceActionBase

class HelloWorldBase(InterfaceActionBase):

    name = 'Hello World GUI'
    author = 'The little green man'

    def load_actual_plugin(self, gui):
        from calibre.gui2.actions import InterfaceAction

        class HelloWorld(InterfaceAction):
            name = 'Hello World GUI'
            action_spec = ('Hello World!', 'add_book.png', None,
                    _('Ctrl+Shift+H'))

            def genesis(self):
                self.qaction.triggered.connect(self.hello_world)

            def hello_world(self, *args):
                from calibre.gui2 import info_dialog
                info_dialog(self.gui, 'Hello World!', 'Hellooo World!',
                        show=True)

        return HelloWorld(gui, None)

Add this plugin and press Ctrl+Shift+H in the gui to see a nice Hello World popup