The World Clock widget is fine, it does what it says, but it can easily be made a little better. Like including your home town, or the town where your family is, rather than the fairly generic ones Apple includes. To do this, you need to edit two files. Start by control-clicking on /Library: Widgets: World Clock.wdgt, and choosing Show Package Contents from the pop-up menu. In the top-level folder that just opened, you need to edit WorldClock.js

Open it up in you favourite text editor, and find a line with a city in the time zone you'd like to use. Duplicate that line, and change it to your desired town. I searched for Wellington in WorldClock.js, duplicated it, and changed the new line, so it now looks like this:

 {city:'Wellington', offset:720, timezone:'Pacific/Auckland'},
 {city:'Auckland', offset:720, timezone:'Pacific/Auckland'}

Note the comma after the first line. This change adds Auckland to the drop down list on the back of the widget. Next, navigate into the English.lproj (or whatever language you're using) folder, and open localizedStrings.js. Once again, I found Wellington, and added the second line, as shown below:

  localizedCityNames['Wellington'] = 'Wellington';
  localizedCityNames['Auckland'] = 'Auckland';

The bracketed name must be the same as the name in WorldClock.js. The name on the right is what World Clock will display below the time. The folders that hold these files are read-only, so I tend to save the modified files to the Desktop, and then drag them back into place and Authenticate as an admin.

Next time you restart Dashboard or create a World Clock, the new town(s) will be available.

[robg adds: You may wish to create a backup before you start, especially if you're not used to mucking about with application bundles...]