I am in contact with people all over the world and I often find myself wondering if they are at work, or at least awake, should I need to email or IM them.  After trying several utilities that would show multiple time zones, I finally found one while listening to an episode of Hanselminutes, that I would recommend.  Mostly because it's free and keeps out of the way.

Microsoft Time Zone.

After being installed and configured, it runs as a Tool Tray application.  Clicking on the icon, produces something similar to this: