Below is a geochron, showing the regions of the Earth's surface in daylight and in shadow. I originally wrote this Java applet for my father, so he could figure out when he could phone me once I moved to the United States! Source code for the applet can be found here.
Note that the date and timezone of your computer must be set correctly in order for the applet to calculate the current Greenwich Mean Time.
Technical note: the applet doesn't seem to be working properly in Internet Explorer. I'm looking into this at the moment (2009-02-07).