This is the most featureful treo clock app I've ever seen. It includes a calendar, alarms, timers, stopwatch functionality, world time display, moon phase, and more. The "wireless edition" includes the weather and currency exchange rates. Currency exchange rates? Do you need this from your clock? For a $7 premium ($20 total), why not?

Plus, their web site offers numerous skins that you can customize with your own background pictures.