I have always been fascinated by the elegance of globe map representations showing the zones lighted by the Sun or left in the night. Probably because the longues undulating sinusoids remind me of when I was still able to play with mathematical equations (I am quite rusted now). I tend to easily notice such a representation and -sometimes- I can use them.
First, Google maps lead to nearly everything and anything: DayLightMap uses them to build a very nice time visualization on the surface of the globe. A nice combination of satellite images taken by day and by night. Very elegant.
And I always have with me a small application software for my Sony CliÃ© PDA: CityTime from CodeCity provides an easy way to to visualize the hours on the surface of the globe and to precisely determine the hour on specific locations/cities.
CityTime exists in versions for PalmOS as well as for PocketPC.