Windows 7 include an enhancement to the system clock that lets you display 2 additional clocks when you hover your mouse over the system clock in taskbar.

This is how it looks like when you enhance your system clock and hover your mouse over the system clock

Follow following instruction to add additional clocks to your windows 7 taskbar.

Step I - Right-click on the clock in the system tray, and click on Adjust Date/Time as shown below

Step II - Click the Additional Clocks tab as shown below

Step III - Select show this clock and select your prefered  time zone. You can also "Enter display name" of your choice. Click on OK to save the changes.