Although the included Time app from Apple is useful enough, Time Zones + World Clock Time Converter (free with in-app purchase) goes Apple a few steps better.
The attractive GUI features the ability to rename locations, so you can call a place "Mom and Dad". The app displays the time differential in hours for all the locations you enter, and a 'quick check' mode lets you look up a date and time anywhere in the world, converting it to your local time. I could see that being useful for business calls in different time zones.
As the day moves along, the colors of the GUI change to reflect the time of day.