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.

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.

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