Enable Additional Clocks in Windows Vista

Windows Vista includes an enhancement to the system clock that lets you
display 2 additional clocks when you hover your mouse over the system clock.

Here's an example of the London clock:

Adding these additional clocks is extremely easy. First right-click on the
clock in the system tray, and choose Adjust Date/Time:

Click the Additional Clocks tab, and select the options and time zone as
shown:

You can enter a friendly display name. I entered "London", but you could put
something relevant to you, like the name of a family member living elsewhere.