Windows 7 and Vista include 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 in Vista:

And here’s the hover clock in Windows 7:

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.