World Ticker is an invaluable tool for anyone with clients in other time zones, especially if they are halfway across the world. By adding clocks from various regions to your menu bar, you can easily keep track of the time in any of a few hundred locations around the globe, and with the number of settings installed with the app, you can manage how it is displayed with ease.
Installation of World Ticker requires you to unpack a DMG and move it to your Applications folder. If you are using Mountain Lion, you'll need to authorize the app, as it is not distributed via the App Store. Once this is done, however, the app moves to your menu bar where you can turn it on permanently and then install each of your world clocks. Adding new clocks is easy, and allows you to choose from a couple hundred options on the drop-down menu. You can then select how the time displays, whether it is labeled, and how it will interact with you and your work. It's mostly a display tool, but it's a well-made one.
If you work with anyone overseas, are constantly forgetting what time it is in different cities, or just enjoy seeing what time it is in places far from where you are, World Ticker will do the trick. It's not a particularly advanced tool but it does exactly what it advertises and it runs quietly in the background while you work.