For me, it would be the following:
Ilium Software's Keep Track and
Phatware's Calligrapher.
If I lost either of these, I wouldn't carry my PDA with me hardly at all.
KeepTrack saved my checking account from a fate worse than death; and Calligrapher allows me to quickly take meeting notes with my own hand writing right in Pocket Word.
As KeepTrack is a personal app, and Calligrapher is used primarily at the office, both these apps make my iPAQ 3765 an all around winner.
Christopher Spera
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pocketnow Writer
pocketnow.com -- it's all about portability...
http://www.pocketnow.com