QGIS » QGIS Application

the latter works. The problem is that your tool is destroyed before you get a chance to use it. Following works:

# Import the [[PyQt]] and QGIS libraries
from [[PyQt]]4.QtCore import * 
from [[PyQt]]4.QtGui import *
from qgis.core import *
from qgis.gui import *

class simplePlugin:

    def +init+(self, iface):
        # Save reference to the QGIS interface
        self.iface = iface

    def initGui(self):
        # Create action that will start plugin configuration
        self.action = QAction(QIcon(":/plugins/totem/icon.png"), "test", self.iface.mainWindow())
        # connect the action to the run method
        QObject.connect(self.action, SIGNAL("activated()"), self.run)
        self.iface.addPluginToMenu("Plugin...", self.action)

    def unload(self):
        # Remove the plugin menu item and icon
        self.iface.removePluginMenu("Plugin...", self.action)
        self.iface.removeToolBarIcon(self.action) 

    def run(self):
        mapCanvas=self.iface.mapCanvas()
        # Create the appropriate map tool and connect the gotPoint() signal.
        self.emitPoint = [[QgsMapToolEmitPoint]](mapCanvas)
        mapCanvas.setMapTool(self.emitPoint)
        QObject.connect(self.emitPoint, SIGNAL("canvasClicked(const [[QgsPoint]] &, Qt::MouseButton)"), self.clickButton)

    def clickButton(self, pnt, but):
        QMessageBox.information(None, "Nome finestra", "cliccato", "&Ok", "&Cancel",  "", 0, 1)