XKCD Now is a useful tool that puts time zones into perspective by visually showing you what time it is in all the different parts of the world. The world map rotates with the actual time to display times around the world and when the daylight hours are.

Now redux-ed to give you a way better experience on Windows!
Acrylic-ish background gives emphasis to the beauty and simplicity of the clock.
You can freely drag the ‘world’ around to see what time and if it is rude to call on a different place.
Tap twice or double click to reset the clock to the current time.