I've seen a few Google Maps apps over the year that allow you to click on a map and get the current time at that location. Timezone Picker, however, is the first one I've seen that shows you all the countries in each time-zone.
The code for TimeZone Picker is available on GitHub, alongside links to two demo maps. One demo uses the Google Maps API and the other uses OpenLayers. On both maps if you click on a country you can view the current time at that location and a shape file of the whole time-zone.