If you're a frequent traveler then keeping track of time zones or changing your clock manually is both inconvenient and a bit risky, especially if you set it wrong, aren't aware if daylight saving rules apply etc.
If you're a Firefox user, FoxClocks is an extension for Firefox, Thunderbird and Sunbird that displays world times in your statusbar or toolbar. You can customise it in any way you like such as by making clocks for specific regions turn green when it's a sociable time to call people in that region. You can browse time zones by country, region or city but the important thing is to enable automatic updates which will ensure that you'll always have the right time.
If you don't like the format of the time in the statusbar, change it: choose from a set of useful standard formats, or create your own - custom formats are easy to create and very flexible. Probably the coolest feature of FoxClocks is the option to use Google Earth to take you to any location in its database (which however doesn't cover every conceivable country in the World), right-click on it and it will open the location in Google Earth.
FoxClocks is a useful tool for anyone who travels a lot or simply wants to improve their geographical knowledge.