Clocker

My job is very time zone intensive, we run events in many parts of the world. We also have regional offices in various locations, and my team is largely remote and scattered throughout the globe.

On my Mac, I utilize Clocker to keep track of the time across all these places. It runs as a menu bar item, is highly configurable (You can see my setup) and can also live in its own window on my desktop. In addition to the display of the various times, you're able to see your next meeting or two.

Highly recommend this software if you manage a lot of time zones.