Palm Applications I've used

Reference

Palm Bible+
Palm Bible+ - The Palm Bible+ program allows you to install multiple versions of the Bible on your PDA. It allows easy navigation to passages and full text searching of the entire Bible.
Noah Pro
Noah Pro is a free dictionary for your palm pilot. It contains most words that I look up and having it easily available on my PDA is very nice!
ArsLexis Thesuarus
"ArsLexis Thesuarus is an English thesaurus offering definitions of 48 thousand words, support for external memory cards such as SD/MMC card or Memory Stick, memory-saving high compression of data, high-resolution screens, latest Palm OS 5 devices and Treo 600/650 and Tungsten 5-Way navigator. "

A5
A5 - A5 is a very configurable program which will show the items from your Date Book and To Do list in a list which can be configured to look many different ways. I use it almost every day!
Mañana
Mañana - Mañana is a To Do list clone which allows you to keep two separate To Do lists. I used to use Mañana to keep a list of prayer requests on my PDA.
HappyDays
HappyDays - HappyDays allows you to keep track of upcoming birthdays and other special events. It provides a nice list with the age that people will be turning on their birthday. It also can be configured to display reminders on your Date Book. It's a very nice program.

FreeCoins
FreeCoins - FreeCoins is a powerful program which allows you to keep track of your money. You can keep track of multiple accounts and transfer funds between them. I find it much easier to use than a paper system because it will automatically calculate your balance and because I almost always have my PDA right there in my pocket. I use FreeCoins to keep track of all my money.