Datebk - replacement calendar for Palm devices. This app. has kept me from straying from the Palm universe for 5 or 6 years. It supports (customizable) icons, a broad array of recurrence options, integrated todos with calendar (not offered by the original Palm calendar app), templates for appointments, links between appointments, todos, addressbook entries, and notes, purging/archiving of old appointments, snoozing alarms, time zones, hiding appointment categories by day/time, floating appointments (move from day to day until checked off), a slew of data entry short-cuts.
I often find it easier to enter new appointments on my Treo (and did with my graffiti-using Palms before) than on my notebook using Outlook or Entourage.
The user support offered by the application's creator, CES Dewar, is the best that I have ever experienced.
And if all that weren't enough, all the proceeds from the purchase of the product go towards the upkeep of a gorilla habitat that Dewar (independently wealthy from selling his desktop publishing business) has established in the mountains of Northern Georgia.