I heart Gnome’s international panel clock
This might be an old feature, but its one I've found wildly useful: the ability to set locations in the panel clock. Other desktops let you do this, but it's all the little extras that Gnome does. Perhaps KDE 4.2 does, I haven't had the chance to try yet.
When you define a series of locations they appear on a map of the world complete with an approximate night/day cast. Under the map each location is shown with digital and analogue clocks, and what I've found absolutely brilliant is each timezone is displayed relative to where you are, NOT to GMT! For example I can see Singapore is 1:30 behind us here in Adelaide, and Jim Kloss is 17:30 behind.
By default it's turned off, but if you use Gnome click the clock on your panel and click Locations.