For the last several years I have worked daily with folks in the United Kingdom and California. When communicating about times you must always include either timezones -- e.g. EST, PST, GMT (I can't bring myself to use UTC) -- or time pairs -- e.g "11:52 AM / 2:25 PM" -- in the conversation. To help keep my sanity I use FoxClocks to show in the Firefox status bar the times for each zone.
A really useful enhancement would be to be able to select one of the zone displays, enter a time, and have the other zone displays show me the time in each of their zones. For example, in this mockup I have ticked the "Peace Dale" time, entered "9 am", and FoxClocks would display the times for San Francisco and Oxford.
Perhaps this feature already exists in one of the many desktop widget sets available.
Update: The The World Clock Meeting Planner which allows you to display a timetable configured for serveral locations.